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QP34.M78  1899     Elementary  physiolog 


LEJ 

RECAP 


Physiology 


MOORE 


Columbia  IHnibersitp 

in  tfte  Citp  of  igeU)  |?orfe 


COLLEGE  OF  PHYSICIANS 
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ELEMENTARY    PHYSIOLOGY 


ELEMENTARY 

PHYSIOLOGY 


BY 


BENJAMIN    MOORE,    M.A. 

PROFESSOR    OF    PHYSIOLOGY    IN    THE   IVIEDICAL    DEPARTMENT    OF    YALE    UNIVERSITY" 

LATE    SHAKPEY    RESEARCH    SCHOLAR,    AND    ASSISTANT    PROFESSOR    OF 

PHYSIOLOGY,    AT   UNIVERSITY   COLLEGE,    LONDON 


JVITH    ONE    HUNDRED    AND    TWENTY-FIVE 
ILL  USTRA  TIONS 


NEW   YORK 
LONGMANS,    GREEN,    AND    CO. 

LONDON   AND    BOMBAY 
1899 

All  rights  reserved 


PREFACE 

This  book  is  intended  to  give  an  idea  of  the  structure  of  the 
body,  and  of  the  changes  which  are  constantly  taking  place  in 
it  during  life,  to  those  who  have  no  previous  knowledge  of  the 
subject. 

It  has  accordingly  been  written  in  as  elementary  a  fashion, 
and  with  the  use  of  as  few  technical  terms,  as  possible,  so  as  to 
be  intelligible  to  the  general  reader. 

The  subject  is  such  a  vast  one  that  an  outline  sketch  is  all 
that  can  be  presented  in  a  volume  of  the  dimensions  of  the 
present  one,  but  it  is  hoped  that  this  sketch  will  be  sufficiently 
clear  to  give  a  fair  idea  of  the  subject  to  the  student  of 
Physiology  before  he  proceeds  to  more  detailed  study  with  the 
aid  of  larger  text-books. 

Such  a  bird's-eye  view  of  the  subject  as  is  here  shown  ought 
to  prove  of  advantage  to  the  junior  student,  who  is  often 
plunged  into  a  mass  of  detail,  and  gets  so  involved  in  this  that 
he  loses  sight  of  the  main  outstanding  features  of  the  subject. 

At  the  same  time,  it  is  hoped  that  the  book  may  attract  a 
larger  circle  of  readers  and  may  remove  some  of  that  deplor- 
able ignorance  which  is  so  often  met  with,  even  among  fairly 
well  educated  people,  as  to  the  general  structure  of  their  own 
bodies  and  the  actions  which  take  place  within  them  during 
life. 

Physiology  is  a  subject  with  which  a  living  acquaintance  can 
only  be  made  by  employing  experimental  methods,  and  hence 


vi  Preface. 

it  is  very  desirable  that  the  student  should  as  far  as  possible 
perform  for  himself  the  experiments  described  in  the  text.  It 
is  further  to  be  hoped  that  doing  so  will  train  him  to  habits  of 
observation,  and  awaken  his  interest  so  that  he  will  be  able  in 
the  end  to  see  for  himself  things  to  which  his  attention  is  not 
specially  directed  by  others. 

In  conclusion,  I  desire  to  thank  my  friend  Mr.  D.  P. 
Rockwood  for  his  kindness  in  revising  the  proof-sheets  and 
preparing  the  index.  Most  of  the  illustrations  have  been 
selected,  by  kind  permission  of  Prof.  Scliafer,  from  the 
"Essentials  of  Histology,"  and  from  Quain's  "Anatoniy." 


B.  MOORE. 


University  College,  London, 
October,  1898. 


CONTENTS 


CHAPTER  I'AGE 

1.    Introduction i 

II.     The  Skeleton  and  its  Articulations 13 

III.  The  Muscular  System 56 

IV.  Position  of  the  Viscera 79 

V.    The  Circulatory  System 96 

VI.    The  Blood 121 

VII.     Diet,  Digestion,  Absorption,  and  Metabolism     .     .  130 

VIII.     Respiration 173 

IX.     Animal  Heat ,     .     .      .....  192 

X.     Excretion , 198 

XI.    The  Nervous  System  .     .     ,     , 218 

XII.     The  Senses 245 

Appendix , 275 

Test  Questions •  285 

Index -     .- 289 


ELEMENTARY    PHYSIOLOGY 


CHAPTER  L 

INTR  OD  UCTION. 

Physiology  is  the  study  of  the  changes  which  go  on  in  living 
matter.  Throughout  the  entire  life  of  any  living  creature  various 
changes  are  constantly  taking  place,  and  it  is  by  means  of  the 
impressions  conveyed  to  our  senses  by  these  changes,  which 
awaken  other  changes  in  us,  that  we  are  made  aware  of  the  life 
of  the  creature. 

The  forms  of  life  which  we  can  recognize  in  the  world 
around  us  are  countless  in  their  variety,  but  all  these  forms 
have  this  in  common  that  a  store  of  energy  is  taken  in  from  the 
outer  world  in  various  ways  and  used  by  the  living  creature  to 
carry  out  the  different  acts  or  changes  which  constitute  its  life. 

In  a  broad  sense  of  the  word,  physiology  includes  the 
study  of  the  life  of  plants  as  well  as  that  of  animals,  but  plant 
life  is  the  special  province  of  the  botanist.  The  physiology  of 
the  lower  forms  of  animal  life  is  also  usually  left  to  the  care  of 
the  zoologist.  We  shall  hence  devote  the  greater  part  of  the 
space  at  our  disposal  to  giving  an  outline  of  the  physiology  of 
man  and  of  those  animals  which  are  closely  allied  to  man  in 
their  structure. 

It  will  be  well,  however,  before  doing  so  to  very  briefly 
describe  the  simplest  type  of  animal  life  with  which  we  are 
acquainted,  in  order  to  obtain  some  general  idea  of  how  this 
simple  form  of  life  is  structurally  related  to  the  more  complex 
animals  which  we  have  subsequently  to  consider. 

7 

/ 


2  Elementary  Physiology. 

The  simplest  form  of  animal  life  with  which  we  are 
acquainted  is  a  minute  speck  of  material  which  is  only  visible 
through  a  microscope.  Small  as  it  is,  it  is  however  a  com- 
plete organic  whole,  capable  of  showing  all  the  essential 
characteristics  of  life,  and  of  producing,  at  what  corresponds  to 
the  end  of  its  existence  as  an  individual  unit,  other  similar 
organisms  which  repeat  again  an  existence  that  is  a  fac- 
simile  of  its  own.  Examples  of  organisms  of  such  a  simple 
type  are  to  be  found  in  the  amoeba  of  pond  water,  and  also  in 
the  leucocytes^  or  whitd  blood  corpuscles^  which  carry  on  a  separate 
but  dependent  existence  during  our  life  in  our  blood.  These 
leucocytes  exist  in  the  blood  of  all  the  higher  animals.  They 
live  in  the  blood  so  long  as  it  circulates  in  the  body,  but  soon 
after  the  blood  is  shed,  or  after  the  animal  dies,  the  leucocytes 
perish. 

When  examined  by  the  aid  of  a  microscope,  each  of  these 
simple  little  organisms  is  seen  to  consist  of  a  mass  of  very 
irregular  outline.  The  material  of  which  it  is  composed  is 
more  or  less  granular  in  appearance,  and  the  granules, 
although  usually  minute,  vary  in  size.  At  one  part  of  the 
tiny  organism  the  material  is  seen  to  be  somewhat  denser  and 
less  transparent  in  structure.  This  denser  portion  is  usually 
rounded  or  globular  in  shape,  and  is  also  more  granular  than 
the  remainder;  it  is  known  as  the  nucleus.^  and  when  the 
organism  is  in  a  resting  position  occupies  a  central  place  in 
the  mass. 

If  this  little  mass  of  living  material  be  carefully  and 
patiently  observed  through  the  microscope,  it  will  be  found 
that  its  shape  does  not  remain  constant ;  there  is  a  slow  change 
going  on  in  its  outline,  and  in  a  short  time  its  appearance  may 
be  quite  different  from  that  which  it  had  when  first  observed. 
The  rate  at  which  the  alteration  in  shape  goes  on  varies  with 
certain  circumstances,  such  as  the  temperature  of  the  fluid  in 
which  the  organism  is  immersed  and  the  presence  of  food 
particles  in  its  immediate  neighbourhood  \  but  usually  the  rate 
is  not  so  sudden  as  to  be  very  obvious  to  the  eye.  It  is  only 
when  the  organism  is  watched  for  a  time  that  the  change  in 
shape  becomes  evident,  and  a  clear  idea  of  how  the  change 


hitroduction.  3 

takes  place  is  best  obtained  by  sketching  the  outUne  at  intervals 
of  a  minute,  or  by  watching  one  of  the  jagged  processes  in  the 
irregular  outline.  It  is  then  seen  that  the  movement  which 
brings  about  the  change  of  shape,  or  it  may  be  a  change 
in  position  of  the  creature  as  a  whole,  is  a  flowing  one.  At 
an  indefinite  point,  a  minute  process  apparently  spontaneously 
grows  out  from  the  main  body;  this  is  at  first  clear  in  its 
interior,  later  it  grows  in  size  and  becomes  granular  inside  from 
an  accession  of  granular  material  from  the  main  body.  More 
than  one  such  process — or,  as  it  might  be  termed,  feeler — is 
usually  to  be  seen  at  any  time  projecting  from  the  main  mass. 


Fig.  I. — Changes  of  form  of  a  white  corpuscle  of  newt's  blood,  sketched  at  intervals 
of  a  few  minutes,  showing  the  inception  of  two  small  granules  and  the  changes 
of  position  these  underwent  within  the  corpuscle. 

The  size  to  which  a  process  can  grow  depends  upon  circum- 
stances; occasionally  after  it  has  reached  a  certain  size  it  is 
again  withdrawn  into  the  main  mass,  as  if  a  determination  had 
been  made  at  some  headquarters  that  nothing  was  to  be  gained 
by  further  motion  in  that  direction.  At  other  times  a  process 
goes  on  increasing  in  size  until  finally  the  whole  of  the  mass 
has  flowed  into  it,  and  the  animal  has  completely  changed  its 
position.  In  many  cases  such  a  continued  growth  of  a  process 
is  due  to  the  existence  of  a  food  particle  in  that  direction.  In 
such  a  case  the  mass  of  the  organism  flows  round  the  particle, 
envelops  it  in  its  mass  for  a  time,  and  afterwards  rejects  any 
unserviceable  debris  that  may  be  left  by  flowing  in  the  same 
slow  fashion  away  from  it. 

The  minute  organism  seems  to  become  aware,   in   some 


4  Elementary  Physiology: 

way,  of  the  presence  of  a  food  particle  even  before  it  has 
touched  it,  possibly  from  minute  traces  of  dissolved  substances 
surrounding  the  particle  of  food  which  become  stronger  as  it 
is  approached.  Various  chemical  substances  determine  move- 
ments of  the  organism  towards  them,  or  away  from  them,  or 
prevent  movement  altogether  and  cause  the  creature  to  assume 
a  spherical  shape.  Electric  shocks  passed  through  the  fluid 
have  a  similar  effect,  the  processes  are  all  drawn  in,  and  the 
creature  becomes  a  rounded  mass.  After  a  time,  if  the  dis- 
turbing influence  be  removed,  the  processes  are  gradually 
formed  again,  and  the  flowing  movement  recommences.  If 
stronger  chemical  reagents  are  used,  or  if  the  fluid  in  which  the 
animal  lives  is  heated  above  a  certain  temperature,  death  is 
the  result.  The  existence  of  one  of  these  tiny  organisms  as  an 
individual  may  terminate  in  one  of  two  ways ;  it  may  die  in 
some  way,  or  it  may  resolve  itself  into  two  smaller  organisms^ 
which  afterwards  increase  in  size  and  repeat  a  life  history  like 
that  of  the  single  organism  from  which  they  originated.  When 
the  organism  is  well  supplied  with  food  it  takes  up  more  than 
is  sufficient  to  supply  energy  for  the  changes  in  shape  above 
described.  The  food  is  changed  within  the  mass,  and  is  in 
part  chemically  altered  and  built  up  into  the  organism,  so  that 
the  bulk  goes  on  increasing  until  a  maximum  size  is  reached. 
Soon  after  this  the  denser  portion,  known  as  the  nucleus,  begins 
to  constrict  at  its  middle,  becomes  hour-glass  shaped,  and 
finally  divides  into  two ;  next  the  general  mass  also  constricts 
a  portion  forming  round  each  of  the  new  nuclei,  and  finally 
there  are  two  smaller  masses  formed,  each  with  a  new  nucleus. 
Each  of  these  new  masses  is  thus  started  into  life  as  a  new 
individual,  and  so  the  cycle  goes  on  repeating  itself. 

When  the  materials  of  which  the  bodies  of  animals  are 
made  up  are  examined  under  the  microscope,  it  is  found  that 
they  consist  of  aggregations  of  minute  units  which  are  related 
in  their  nature  to  the  simple  organism  which  has  been  described 
above.  These  microscopic  units  are  spoken  of  as  cells.  Each 
cell  typically  consists  of  a  minute  mass  of  semi-fluid  material 
cdiW^di  protoplasm.  At  one  part  this  protoplasm  is  different  from 
the  remainder ;  it  is  here  denser  and   more    complex   in   its 


Introduction.  5 

structure,  and  is  known  as  the  nucleus.  The  nucleus  seems  to 
be  the  most  highly  vitalized  portion  of  the  mass,  and  any  im- 
portant changes  in  the  life  of  the  cell,  such  as  its  division  into 
two  new  cells,  commence  first  in  the  nucleus.  Granules  are 
often  visible  within  the  cell  protoplasm ;  these  are  not  part  of  the 
protoplasm  in  the  sense  that  they  are  not  living,  but  are  usually 
composed  of  material  which  has  been  made  by  the  protoplasm 
from  the  food  on  which  it  has  acted.  This  material  is  after- 
wards used  for  carrying  on  various  operations  which  make  up 
the  life  of  the  cell. 

Thus  each  living  cell  in  the  animal's  body  resembles  its 
fellows,  in  that  it  takes  in  certain  substances  in  the  form  of  food, 
and  changes  these  substances  chemically  into  other  substances, 
or,  as  it  is  called,  assimilates  them.  These  new  substances 
serve  various  ends.  For  example,  some  supply  a  store  of 
energy  to  the  cell  to  carry  on  its  own  life ;  others  are  turned 
out  again  to  form  nutriment  to  other  different  cells  in  the  body; 
others,  again,  stream  off  in  solution  to  act  on  the  food  of  the 
animal,  and  render  it  capable  of  absorption  by  another  class  of 
cells. 

The  cells  which  go  to  make  up  the  bulk  of  an  animal  vary 
enormously  in  their  appearance.  Only  a  very  small  percentage 
are  capable  of  freely  moving  about,  and  taking  in  food  in  the 
same  fashion  as  the  simple  unicellular  organism  which  has  been 
described  above  as  a  type.  By  far  the  greater  number  are  fixed 
in  shape  and  position ;  these  must  have  their  food  brought  to 
them  in  solution,  for  they  cannot  absorb  solid  particles ;  they 
must  be  bathed  in  a  nutrient  fluid  which  is  carried  to  them  in 
some  manner,  and  this  nutrient  fluid  must  be  prepared  in  some 
fashion  from  the  solid  food  which  the  animal,  of  which  they 
form  a  part,  eats  from  time  to  time. 

It  is  hence  easy  to  see  that  in  animals  consisting  of  large 
colonies  of  cells  there  must  be  a  division  of  labour ;  one  class 
of  cells  performing  one  class  of  work,  while  another  class  carries 
on  another.  This  purpose  is  best  attained  by  an  arrangement 
by  which  those  cells  which  carry  on  the  same  kind  of  labour 
are  situated  at  the  same  place,  and  such  is  the  arrangement 
which  is  found  in  the  animal  body.     The  cells  are  not  situated 


6  Elementary  Physiology. 

indiscriminately,  but  in  aggregations,  each  consisting  of  a  very 
large  number  of  similar  cells  carrying  out  a  similar  purpose. 
To  these  aggregations  of  cells  different  names  are  given  for 
purposes  of  description,  such  as  tissue^  organ^  gland,  and 
body. 

"  Tissue"  is  the  most  general  of  these  terms,  and  is  applied  to 
any  assemblage  of  cells  designed  for  a  common  purpose.  Thus 
there  is  a  tissue  found  generally  all  over  the  body,  the  purpose 
of  which  is  to  bind  such  aggregations  of  cells  as  we  have  been 
describing  into  bundles  and  masses,  and  to  form  sheaths  and 
coverings  for  them ;  this  is  known  as  connective  tissue. 
Again,  the  muscles  of  the  body  by  which  the  movements  of  the 
animal  are  brought  about  are  made  up  of  cells  which  are 
collectively  spoken  of  as  muscular  tissue. 

The  term  "  organ  "  is  more  restricted  in  its  application,  and 
is  usually  applied  when  the  purpose  served  by  the  mass  of  cells 
is  more  specific  in  its  character.  Thus  the  heart  is  an  organ, 
the  purpose  of  which  is  to  send  the  blood  streaming  round  the 
body,  carrying  nutriment  to  the  cells  of  which  the  other  tissues 
and  organs  are  composed.  Since  it  is  chiefly  composed  of 
muscular  tissue,  it  is  further  called  a  muscular  organ.  An  organ 
may  thus  be  composed  of  several  difterent  tissues,  just  as  a 
rock  may  be  made  up  of  different  minerals.  For  example,  the 
heart  contains  connective  tissue,  and  adipose  or  fatty  tissue,  as 
well  as  muscular  tissue.  Also  a  tissue  may  go  to  form  a  part 
of  very  diverse  organs;  thus  muscular  tissue  is  found  in  the 
skeletal  muscles,  in  the  heart,  in  the  walls  of  blood-vessels,  and 
in  the  coats  of  the  alimentary  canal. 

A  gland  consists  mainly  of  aggregations  of  cells  the  purpose 
of  which  is  to  take  up  certain  materials  from  the  blood,  and 
from  these  to  make  new  materials,  which  are  either  returned  to 
the  blood  to  be  of  service  to  the  animal  at  some  other  part,  or  to 
be  excreted  from  the  body  as  useless  and  injurious  by  some 
other  gland ;  or  are  sent  into  a  collecting  vessel  called  the  duct  of 
the  gland,  dissolved  in  a  watery  fluid  which  is  known  as  the 
secretion  of  the  gland. 

These  gland  secretions  serve  many  purposes  in  the  body. 
Some  are  lubricants,  such  as  the  secretion  of  the  lachrymal 


Introduction.  7 

gland  of  the  eye,  which  serves  to  keep  the  eyeball  moist  and 
float  off  any  irritant  particles  which  may  reach  the  sensitive 
surface  of  the  eyeball.  The  ^///^_/ function  of  the  saliva  secreted 
by  the  salivary  glands  is  similarly  to  coat  the  food  as  it  is 
swallowed  with  a  slippery  envelope,  and  render  its  passage  to  the 
stomach  an  easy  one.  Other  secretions  contain  substances 
dissolved  in  them  which  act  on  the  food  eaten  by  the  animal, 
dissolve  it,  and  render  it  capable  of  absorption  by  those  cells 
which  line  the  passage  called  the  alimentary  canal  by  which 
the  food  passes  through  the  body.  These  secretions  will  be 
considered  more  fully  in  treating  of  digestion. 

In  other  cases  the  secretion  of  the  gland  contains  only 
substances  which  are  either  of  no  service,  or  directly  injurious 
to  the  body,  and  the  purpose  of  the  secretion  is  to  secure  the 
removal  of  these  from  the  circulating  blood-stream,  and  so  away 
from  the  body;  in  such  a  case,  the  process,  though  really  identical 
in  nature  with  that  of  secretion,  is  spoken  of  as  excretion.  The 
best  example  is  that  of  the  kidneys.  Here  substances  which 
exist  ready  formed  in  the  blood,  and  the  accumulation  of  which 
in  the  blood  would  lead  to  the  poisoning  of  the  cells  and  to  the 
death  of  the  animal,  are  kept  down  to  a  normal  level  by  being 
continuously  removed  by  the  secreting  action  of  the  cells  of  the 
kidney.  In  the  case  of  the  liver  the  secretion  {the  bile)  has  a 
mixed  character.  Some  of  the  substances  held  in  solution  are 
useless  to  the  body,  and  are  finally  carried  away  from  the  body, 
mixed  with  the  unused  and  indigestible  part  of  the  animal's 
foods.  Other  substances  secreted  in  the  bile  are  useful  in  the 
process  of  digestion,  and  are  absorbed  again  by  the  cells  lining 
the  alimentary  canal. 

Certain  glands  in  the  body  do  not  possess  ducts,  and  have 
no  obvious  secretion.  Before  their  true  nature  was  known, 
some  of  these  ductless  glands  were  spoken  of  as  bodies.  It  is 
not  yet  known  how  these  ductless  glands  act,  but  it  is  certain 
that  they  exert  a  powerful  influence  over  the  welfare  of  the 
animal,  and  in  the  case  of  some  of  them  their  removal  is 
followed  by  the  death  of  the  animal.  It  is  supposed  that  they 
act  like  glands  in  removing  certain  materials  from  the  blood, 
and  in    elaborating    from    these    materials    others    which    are 


8  Elementary  Physiology. 

returned  to  the  blood-stream  again,  and  serve  useful  and  in 
some  cases  indispensable  purposes  in  the  preservation  of  the 
health  of  the  animal.  A  similar  kind  of  work  is  done  by 
certain  of  the  other  glands  which  possess  ducts.  Thus  it  is 
certain  that  the  formation  of  bile  is  not  the  only  nor  even  the 
main  work  of  the  liver.  This  organ  acts  as  a  storehouse  for  a 
certain  class  of  food,  accumulating  in  its  cells  a  reserve  of 
material  when  the  supply  in  the  blood  which  comes  to  it  is  too 
liberal,  and  doling  this  reserve  out  when  the  supply  slackens 
and  the  other  cells  of  the  body  require  this  kind  of  nutriment. 
In  addition,  the  liver  has  a  modifying  action  on  other  classes 
of  food-stuffs,  but  this  subject  will  be  more  fully  considered 
later. 

It  must  not  be  supposed  that  in  each  of  these  aggregations 
of  cells  that  we  have  been  considering,  there  is  to  be  found 
only  one  kind  of  cell.  It  is  only  meant  that  one  type  of  cell 
preponderates  in  them.  These  cells,  to  do  their  work,  must  be 
suppUed  with  blood,  and  this  blood  must  be  carried  to  them  by 
blood-vessels,  and  away  from  them  by  other  blood-vessels. 
The  walls  of  these  blood-vessels  contain  various  kinds  of  cells 
specially  adapted  to  form  the  walls  of  vessels.  There  is  an 
inner  lining  of  flat  cells  to  give  a  smooth  lining  to  the  wall,  and 
outside  this,  again,  muscle  cells  encircling  the  vessel,  which  by 
their  state  of  contraction  or  relaxation  allow  less  or  more  blood 
to  pass  to  the  preponderating  cells. ^  Again,  the  blood  itself 
contains  cells  of  different  kinds.  Further,  the  whole  mass  of 
cells  must  be  supported  and  bound  together  by  some  of  that 
connective  tissue  mentioned  above.  It  is  hence  to  be  under- 
stood that  in  every  tissue  and  organ  of  the  body  there  is  a 
considerable  number  of  different  kinds  of  cell  to  be  found,  but 
that  the  nature  of  these  cells  corresponds  to  the  kind  of  work 
to  be  performed  by  that  tissue ;  some  being  fundamentally 
concerned  with  the  work  in  hand,  while  others  are  necessary 
accessories. 

Harking  back  again  to  our  simple  type  of  a  unicellular 

^  The  blood  does  not  flow  into  these  cells,  but  flows  past  close  to  them. 
The  manner  in  which  the  cells  are  nourished  by  the  materials  carried  by  the 
blood  will  be  shown  later. 


Introduction.  9 

organism,  and  comparing  it  with  the  cells  which  build  up  the 
body  of  one  of  the  higher  animals,  we  see  that  there  is 
something  essentially  the  same  in  the  two  cases.  In  the  case 
of  the  simple  organism,  one  cell  carries  out  all  the  operations 
of  existence.  It  takes  in  food;  it  assimilates  this  food  and  forms 
materials  to  replenish  the  waste  of  the  cell ;  it  rejects  what 
cannot  be  assimilated  ;  it  moves  about  in  the  fluid  in  which  it 
lives,  and  finally  it  continues  its  kind  by  dividing  and  producing 
new  individuals.  Similarly,  a  living  cell  forming  an  integral 
part  of  a  higher  animal  carries  on  to  a  certain  extent  an 
independent  existence.  It  takes  up  the  food  which  is  prepared 
for  it  and  carried  to  it  in  solution ;  it  assimilates  this  food  and 
maintains  its  living  condition  by  its  aid,  and  it  is  capable  of 
undergoing  cell  division  and  increasing  the  number  of  cells 
resembling  itself.  Only  the  existence  of  this  second  kind  of 
cell  is  dependent  on  the  life  of  the  animal  of  which  it  forms  a 
part;  for  on  the  death  of  the  animal  its  supply  of  food  stops, 
its  means  of  existence  are  at  an  end,  and  in  a  variable  but 
always  short  period  of  time  it  inevitably  dies.^ 

In  return  for  the  nourishment  which  each  cell  receives  from 
the  general  stock  of  circulating  food,  it  performs  certain  services 
which  benefit  the  general  community  of  cells  forming  the  body 
of  which  it  is  a  minute  portion. 

Thus  the  most  simple  type  of  animal  life  differs  from  the 
most  complex  only  in  this — that  in  the  latter  there  is  minute 
division  of  labour,  while  in  the  former  there  is  none.  Accom- 
panying this  division  of  labour,  there  is  naturally  that  adaptation 
of  structure  which  is  necessary  to  suit  the  instrument  to  its 
work. 

The  unicellular  organism  might  be  compared  to  an  in- 
dividual savage  inhabiting  an  uncivilized  region,  and  a  com- 
ponent cell  of  a  higher  animal  to  a  citizen  of  a  civilized  nation  ; 
in  which  case  the  civilized  nation  might  further  be  taken  to 
represent  the  mass  of  cells  which  together  make  up  the  body. 
The  savage  is  able,  after  a  fashion,  to  supply  all  his  wants  for 
himself,   and  carry  on  a  certain    low-grade  existence  without 

'  The   death  of  the  annual  as  a  whole   (general   death)  is  hence  not 
synchronous  with  that  of  the  tissues  (local  death). 


lo  Elementary  Physiology. 

calling  in  the  aid  of  others.     The  civilized  man  usually  does 

one  type  of  work   only,  which  is  useful   to  the  community  at 

large,  and  in  return  for  this  mutually  receives  the  service  of 

his  fellows  by  whom  he  is  virtually  fed  and  clothed  and  supplied 

with  comforts  in  proportion  to  the  value  of  the  work  which  he 

does.     In  a  civilized  country  it  is  further  found  that  work  can 

be  best  and  most  expeditiously  done  when  a  large   number 

combine  to  do  it  in  the  same  place,  and  even  certain  industries 

are  confined  to  certain  towns  and  districts  to  the  exclusion  of 

other  places  and  other  industries.     Similarly  in  the  body,  one 

kind  of  work   is  done  in  one  part,  and  a  different  kind  in 

another.     Again,  the  centres  of  industries  in  a  nation  must  be 

kept  in  communication  with  one  another,  and  supplies  of  raw 

material  and  of  manufactured  articles  must  be  carried  from 

one  to  another.     Similarly,  in  the  body  it  is  necessary  to  have 

a  means  of  communication  and  a  vehicle  of  transport  between 

one  part  and  another,  and  this  is  achieved  by  a  stream   of 

circulating    fluid,  the    blood,    which   carries    suppHes   to    the 

various  parts  and  takes  away  anything  which  it  is  necessary  to 

remove.     Finally,  in  a  civilized  community  it  is  essential  that 

there  should  be  a  governing  intelligence,  and  a  rapid  mode  of 

intercommunication  between  this  government  and  the  various 

parts.     Likewise,  in  the  body  it  is  necessary  that  there  should 

be  a  controlling  centre  or  centres  which  can  be  rapidly  made 

aware  of  any  change  in  the  condition   of  any  part.     This  is 

the  office  of  the  nervous  system^  consisting  of  a  central  part, 

the  brain  and  spinal  cord,  which  by  an  immense  network  of 

communicating    channels,    the    nerves.,    is    placed    in    minute 

acquaintance  with    the  state  of  affairs  throughout  the  body, 

and  to  a  certain  extent  in  a  position  to  control  any  changes. 

Between  the  simplest  type  of  living  creature  and  the  most 
complex  type  there  are  an  infinite  number  of  intermediate 
stages,  so  that  it  is  possible  to  see  how  the  more  complex 
species  are  related  to  the  simpler.  The  same  thing  is  clearly 
illustrated  in  the  development  of  an  individual  animal  of  one 
of  the  higher  types.  Every  animal,  no  matter  how  compHcated 
its  final  structure,  commences  life  as  a  single  cell  of  microscopic 
dimensions.     This  primitive  cell,  or  ovum,  is  formed  from  the 


Introduction.  1 1 

body  of  the  female  parent,  and  after  being  fertilized  by  being 
penetrated  by  a  portion  of  a  cell  {spermatozoon)  from  the  body 
of  a  male  parent,  commences  to  subdivide,  the  nucleus  first 
dividing,  as  in  the  case  of  the  simple  unicellular  organism,  and 
afterwards  the  cell  protoplasm.  The  two  cells  so  formed  again 
subdivide,  so  giving  rise  to  four  cells,  and  this  process  is 
repeated  a  great  number  of  times  until  there  is  instead  of  one 
cell  a  spherical  mass  of  many  cells.  The  cells  become  so 
arranged  that  there  are  larger  cells  at  one  hemisphere  than 
at  the  other,  and  by  the  growth  of  one  set  of  cells  over  the 
other  there  are  gradually  formed  an  upper  and  under  layer. 
Later  there  is  developed  between  these  two  layers  a  middle 
layer,  and  from  these  three  primitive  layers  the  various  organs 
take  origin — the  alimentary  canal,  digestive  glands,  and 
respiratory  system  from  the  inner,  the  bony  skeleton  and 
muscular  system  from  the  middle,  and  the  skin  and  nervous 
system  from  the  outer.  As  the  three  layers  are  formed  the 
spherical  mass  elongates,  and  along  one  side  a  groove  is 
formed  by  a  dipping  in  of  the  surface ;  this  groove  deepens 
and  finally  meets  and  closes  at  the  top  so  as  to  form  a  canal. 
Around  this  canal  the  spinal  cord  develops,  and  it  is  in  this 
manner  that  the  centre  nervous  system  comes  to  be  developed 
from  the  external  primitive  layer. 

To  give  even  a  complete  outline  of  the  manner  in  which 
development  goes  on  to  the  final  production  of  the  perfect 
animal  would  take  much  more  space  than  is  occupied  by  this 
entire  volume ;  the  above  is  merely  intended  to  indicate  roughly 
how  the  operation  of  development  of  any  animal  commences 
from  a  single  cell.  During  the  process  of  development  the 
growing  embryo  receives  nutriment  from  the  parent,  so  that  it 
can  grow  and  increase  in  bulk.  This  takes  place  by  means  of 
a  vascular  network  called  the  placenta,  in  which  blood-vessels 
from  the  parent  ramify  alongside  blood-vessels  from  the  embryo. 
The  nutrient  material  passes  from  the  parent  by  a  process  of 
diffusion  into  the  blood  of  the  embryo,  and  the  products  of 
excretion  of  the  embryo  pass  in  the  opposite  direction.  In 
this  manner  the  life  of  the  embryonic  animal  is  maintained 
until  its  development  is  sufficiently  advanced  for  it  to  carry  on 


12  Elementary  Physiology. 

an  independent  existence,  and  soon  after  this  stage  is  reached 
it  becomes  separated  from  the  parent. 

Every  animal,  no  matter  how  comphcated  its  structure, 
then,  may  be  looked  upon  as  an  aggregation  of  physiological 
units  called  cells.  These  cells,  though  formed  on  a  com.mon 
pattern  and  produced  originally  from  a  single  cell,  differ  from 
one  another  in  detail  of  structure,  in  shape  and  in  size,  and 
carry  on  different  functions  in  the  body.  The  work  of  all 
these  cells  is  designed  towards  one  end,  the  welfare  of  the 
whole  body,  and  hence  the  animal  though  made  up  of  an 
immense  number  of  living  units  may  still  be  regarded  as  a 
single  organic  whole,  and  as  possessing  a  life,  as  a  whole,  on 
which  the  Hves  of  the  component  units  depend. 


CHAPTER  II. 

THE  SKELETON  AND  ITS  ARTICULATIONS} 

The  framework  on  which  the  body  is  built  is  called  the 
skeleton.  The  skeleton  consists  in  all  of  about  two  hundred 
bones,  variously  joined  together  so  as  to  afford  protection  and 
support  for  the  soft  parts,  to  give  stability  to  the  shape  of  the 
body,  and  to  render  possible  all  those  complicated  movements 
which  go  on  during  life.  The  bones  are  fitted  together  at  the 
joints,  or  articulations  ;  those  surfaces  which  come  into  contact 
at  the  joints  being  known  as  the  articular  surfaces.  At  some 
of  the  articulations  a  great  freedom  of  movement  is  permitted, 
in  others  the  amount  of  movement  is  but  slight,  and  in  others, 
again,  the  opposed  surfaces  are  firmly  interlocked,  and  no 
movement  is  possible. 

The  bones  are  living  structures  provided  with  a  supply  of 
blood  for  their  nutriment  and  containing  living  cells.  Between 
the  organic  matter  of  which  the  cells  are  composed,  and  other 
organic  matter  directly  derived  from  the  cells,  there  is  deposited 
a  large  amount  of  inorganic  material,  chiefly  consisting  of  the 
insoluble  phosphate  of  calcium  (tri-calcic  phosphate).  This 
inorganic  matter  is  separated  from  the  blood  during  growth  by 
the  cells  which  are  engaged  in  forming  the  bone.  It  forms 
about  two-thirds  of  the  weight  of  the  dried  bone,  and  is  that 
factor  in  the  materials  constituting  the  bone  which  gives 
hardness  and  strength  to  the  structure. 

^  The  student  should  follow  this  description  with  the  aid  of  a  set  of 
dried  bones  ;  or,  if  these  are  not  at  his  command,  should  attempt  to  gain 
access  to  a  museum  where  he  can  study  the  skeleton  and  see  the  bones  for 
himself. 


14 


Elementary  Physiology 


A  bone  is  not  the  same  in  its  structure  at  all  parts ;  strength 
combined  with  lightness  is  obtained  by  a  compact  firm  structure, 
called  compact  bo?ie,  being  formed  all  over  the  outside,  inclosing 
a  space  which,  in  the  case  of  some  bones,  such  as  the  ribs  and 
ends  of  the  long  bones  of  the  limbs,  is  filled  with  a  honey- 
combed, lighter  structure  {cancellous  bone),  and  in  the  case  of 
other  bones,  such  as  the  shafts  of  the  limb  bones,  is  completely 
hollow,  and  filled  only  with  a  soft  fatty  tissue,  the  mairow  of 
bone.     The  purpose  is  similar  to  that  which  causes  the  engineer 


Fig.  2. — Transverse  section  of  compact  tissue  (of  humerus).     (Sharpey.) 
f  Magnified  about  150  diameters.) 

Three  of  the  Haversian  canals  are  seen,  with  their  concentric  rings,  or  lamellae;  also  the 
lacunae,  with  the  canaliculi  extending  from  them  across  the  direction  of  the  lamellae. 

to  employ  hollow  pillars  and  girders  when  he  designs  obtaining 
the  greatest  strength  with  the  least  weight  of  material. 

At  the  ends  or  surfaces  of  the  bones,  where  they  join  or 
articiUate  together,  there  is  found  a  layer  of  tissue  known  as 
cartilage,  or  gristle.  This  tissue  is  closely  allied  in  nature  and 
mode  of  origin  to  bone  itself,  being  indeed  the  kind  of  tissue 
in  which  the  forms  of  growing  bones  are  first  laid  down,  and 
which  is  afterwards  transformed  into  bone.     In  the  layer  of 


The  Skeleton  and  its  Articulations. 


15 


cartilage  at  the  articular  surfaces  no  deposition  of  calcium 
salts  takes  place,  and  no  after  metamorphosis  into  bone ;  this 
layer  retains  throughout  life  that  structure  which  the  whole 
bone  initially  had,  and  serves  an  important  function  in  pre- 
serving the  bones  and  the  whole  body  from  injury  by  any 
sudden  jerks  or  jars.  Cartilage  is  the  buffer  tissue  of  the 
body,  and  prevents  a  sudden  knock  at  any  part  from  being 
transmitted  through  the  body.  Cartilage  is  extensible  and 
compressible  in  the  same  sense  as  a  material  like  indiarubber  : 


isOc^' 


a 


J^j         c3e>        ■siK'.gE?  <3®&> 


^3 1=  .g  c-e. 

■5  ^  E{»  %l 


Fig.   3. — Vertical  section  of  articular  cartilage  covering  the  lower  end  of  the  tibia, 
human.     (Magnified  about  30  diameters.) 

a,  cells  and  cell-groups  flattened  conformably  with  the  surface  ;  b,  cell-groups  irregularly 
arranged  ;  c,  cell-groups  disposed  perpendicularly  to  the  surface  ;  d,  layer  of  calcified 
cartilage ;  e,  bone. 

that  is  to  say,  it  can  easily  be  forced  out  of  shape  by  pressure 
or  traction  in  any  direction,  and  after  the  deforming  force  is 
removed  returns  at  once  to  its  original  shape.  Hence  it  is 
used  throughout  the  body  to  give  elasticity  to  the  movements 
and  to  prevent  injury  by  jarring.  For  this  reason  there  is  a 
pad  of  it  between  each  of  the  separate  bones  which  go  to  form 
the  backbone,  or  vertebral  column^  and  also  on  the  end  of 
each  of  the  limb  bones.     For  a  similar  reason  the  ribs  end  in 


jg  Elementary  Physiology. 

cartilage  so  as  to  give  a  flexible  junction  between  these  and 
h  breast  bone,  or  sternum,  in  front,  and  so  drmrn.sh  the 
fragility  of  the  bony  structures  which  protect  the  upper  cavity 
ofihe  trunk,  known  as  the  chest,  or  tkora..  The  surfaces  of 
the  articular  cartilages  in  those  joints  where  there  is  consider- 
able freedom  of  motion,  such  as  those  of  the  limbs,  are 
exceedingly  smooth,  so  that  another  office  of  these  cartilages  is 
to  prevent  friction  of  bearings  at  the  joints.     In  this  work  the 


F>0    4  -Sagittal  s.cUo„  of  the  temporo.„,axillary_ articulation  of  th,  right  side, 
t  IG.  4.— sagvi.i.di  =  (Allen  Thomson.)    \ 

.  is  placed  dose  .0  the  articular  eminence   and  P^o;-^'y^-",SSy°lVo7rLT7aw, 
pSstSor  portion  of  the  interamcular  disc. 

smoothness  of  the  cartilaginous  surfaces  is  assisted  by  a  fluid 
S  fs  Lreted  at  the  joint,  and  is  called  ^^^^  ^-^■ 
This  synovial  fluid  is  ^^^\^J°^^'ZlTo^^^  ^ 
Eed""  s;tr.~lous^  =,  Ibich  completely 
rounds  the  joilt,  ^onp^^^jf^  — "a 
synovial  fluid  is  secreted.     Besides  lo™i"g  immensely 

kind  of  gear-case  for   the   i^^^X'Tsul^^T^^^ 
strong    and    contributes    materially  to   the   strengi 
^Sre  and  to  keeping  the  bones  in  position  (....  from  getting 


TJie  Skeleton  and  its  Artienlations.  17 

out  of  joint).  In  this  it  is  assisted  at  important  joints  by 
stronger  fibrous  bands,  occurring  either  as  local  thickenings  of 
its  wall,  or  quite  distinct  from  it ;  these  structures  are  known  as 
liganients. 

In  some  important  joints,  such  as  the  knee  joint  and  that 
of  the  lower  jaw  with  the  skull,  the  junction  of  the  two 
articular  surfaces  is  made  more  perfectly  fitting  by  the  inter- 
position of  a  thin  disc  of  cartilage  between  the  two  opposed 
surfaces,  so  that  there  is  a  kind  of  double  joint,  each  opposed 
surface  moving  over  an  opposite  surface  of  the  interarticular 
disc.  The  interarticular  disc  is  attached  round  its  margin  to 
the  capsule  of  the  joint,  and  in  this  way  two  cavities  are 
formed,  which  may  either  be  completely  (jaw)  or  incompletely 
separated  (knee). 

The  foregoing  figures  show  the  minute  structure  of  bone 
(Fig.  2)  and  of  articular  cartilage  (Fig.  3)  as  these  are  seen 
when  thin  sections  of  these  tissues  are  prepared  and  examined 
with  the  microscope.  A  section  through  the  articulation  of  the 
lower  jaw  is  reproduced  as  an  example  of  the  structure  of  a 
joint  (Fig.  4). 


The  Vertebral  Column. 

The  central  structure  in  the  framework  of  the  body  is  the 
backbone,  or  vertebral  column,  which  lies  in  the  mid  line  down 
the  back.  It  may  be  looked  upon  as  a  central  axis  of  the 
skeleton  to  which  the  other  parts  are  attached.  It  supports 
the  skull  upon  its  upper  end,  the  ribs  are  attached  to  it 
laterally,  and  at  its  lower  end  it  itself  is  supported  between 
the  hip-bones,  through  which  the  weight  of  the  body  is  trans- 
mitted to  the  legs. 

The  structure  of  the  vertebral  column  is  shown  in  the 
accompanying  figures.  It  consists  of  a  series  of  bones,  called 
vertebrcB,  usually  twenty-six  in  number  in  the  adult,  which  are 
articulated  together  by  intervertebral  discs  of  cartilage  in  such 
a  way  that,  although  there  is  very  little  movement  between  any 
consecutive  two,  yet  the  column,  as  a  whole,  is  flexible,  and 

c 


i8 


Elementary  Physiology. 


admits  of  a  considerable  amount  of  bending,  both  from  before, 
backward,  and  from  side  to  side  (see  Fig.  7). 


^^t^ 


>^ 


A  A 


fi 


cov 


Fig.  5.  —  The  vertebral 
column,  viewed  from 
the  left  side. 


Fig.  6. — The  vertebral 
column,  viewed  from 
behind. 


CI,  first  cervical  vertebra;  di,  first  dorsal  vertebra;  i.i,  first  lumbar  vertebra  ;  Si,  first 
sacral  vertebra  ;  coi,  first  cocc3'geal  vertebra. 


The  Skeleton  and  its  Articulations. 


19 


With   the   exception    of   the    two    upper    and    two    lower 
members  of  the   series,    which  will    be  considered  later,  the 


Fig.   7. — Section  through  two  lumbar  vertebrae  showing  the  arrangement  of  the 
intervertebral  disc.     (R.  Quain.)     \ 

I,  2,  the  fibrous  laminae  ;  3,  the  central  soft  substance  ;  the  capsule  of  the  joint  between 
the  articular  processes  is  also  shown. 

vertebrae  bear  a  close  resemblance,  or  homology^  to  one  another, 
and  hence  a  description  of  one  typical  vertebra  will  suffice 
for  all. 

The  front  portion  of  each  vertebra  is  a  short  flat  cylinder, 
or  disc,  called  the  body^  a  little  smaller  in  its  middle  than  at 
the  top  or  bottom.  From  the  posterior  part  of  the  body  the 
neural  arch  arises  by  two  short  stout  processes  of  bone  called 
the  pedicles.  These  unite  at  the  back  in  a  broad  flat  plate 
called  the  lamina,  which  projects  backwards  as  the  spinous 
process,  that  part  of  the  vertebra  which  can  be  felt  through  the 
skin  at  the  back.  A  ring  of  bone  is  formed  in  this  way 
surrounding  a  cavity  which  is  known  as  the  spinal  fora7nen. 
It  is  evident  that  when  the  vertebrae  are  joined  together  as  in 
the  body,  these  cavities  will  form  a  long  canal  surrounded  and 
protected  by  bone ;  this  canal  is  called  the  spinal  canal,  and 
lodges  the  spinal  cord,  a  delicate  nerve  structure  which  in  this 
manner  is  preserved  from  any  chance  of  injury.  The  pedicles 
are  narrower  than  the  body,  so  that  there  is  a  roughly  semi- 
circular notch  at  each  side  above  and  below  each  pedicle. 
When  the  vertebrae  are  in  position  in  the  body,  it  is  obvious 
that  there  will  thus  be  at  each  side  between  each  two  vertebrae 


20 


Elementary  Physiology. 


a  round  opening,  half  of  which  is  contributed  by  each  vertebra  ; 
these  openings  are  called  the  intervertebral  foramina^  and  serve 
to  transmit  the  spinal  nerves,  a  pair  of  which  come  off  the 
spinal  cord  at  each  vertebra,  as  well  as  the  blood-vessels, 
carrying  blood  to  and  from  the  spinal  cord  and  its 
accessories. 

From  the  junction  of  pedicles  and  lamina,  two  processes, 
called  the  transverse  processes,  arise,  as  shown  in  the  figure. 


SPINOUS     PROCESS 


RANSVERSE 
PROCESS 


^SUP  AR7IC. PROCESS 
PEDICLE 


Fig.  8. — Tenth  dorsal  vertebra,  from  above.     (Drawn  by  D.  Gunn.) 


In  the  case  of  those  vertebrae  to  which  ribs  are  attached  (dorsal 
vertebrse),  each  of  these  transverse  processes  bears  a  small 
articular  facet,  to  which  a  similar  facet  on  the  corresponding 
rib  is  applied.  On  the  bodies  of  these  same  dorsal  vertebrse 
above  and  below  on  each  side  there  is  a  half  facet,  which  in 
each  case  unites  with  its  neighbour  on  the  nearest  vertebra  to 
make  a  whole  facet,  for  articulation  with  the  head  of  the  rib. 
So  that  each  rib  has  two  points  of  attachment,  one  on  the 
bodies  of  the  vertebrse,  and  one  on  the  transverse  process  of  a 
vertebra. 


The  Skeleton  mid  its  Articulaiions, 


21 


The  lamina  of  the  vertebrae  imbricate  or  overlap  each 
other,  so  that  each  vertebra  has  four  other  articular  surfaces 
besides  those  on  the  body.  This  arrangement  gives  greater 
stability  to  the  column,  and  is  a  safeguard  against  dislocation. 

The  vertebrae  are  further  bound  together  by  strong  bands 
of  ligament  passing  from  one  to  another,  especially  between 
the  spinous  processes  and  between  the  laminae.  The  pro- 
cesses of  the  vertebrae  serve  for  the  attachment  of  certain  of 
the  back  muscles,  which  straighten  the  vertebral  column,  and 
for  some  of  the  muscles  of  the  back  of  the  neck,  which  tilt  the 
head  backwards  on  the  neck. 


POSTERIOR     ARCH 

r        'if ^VERTEBRAL  GROOVE 

ARTIC.  PROC. 


TUBERCLE 


FOR    TRANSV. 
LIGAMCNT 


ANTERIOR    ARCH 
ARTIC.   SURF.   FOR    ODONTOID      PROC. 


Fig.  9. — Atlas,  from  above.     (Drawn  by  D.  Gunn.) 
The  position  of  the  transverse  ligament  is  indicated  by  dotted  lines. 

The  seven  vertebrae  at  the  upper  end  of  the  column  are 
known  as  the  cervical  vertebrae ;  these  form  the  bones  of  the 
neck.  They  are  much  more  slenderly  built  and  lighter  than 
the  lower  members  of  the  column,  and  are  capable  of  moving 
on  one  another  to  a  much  greater  extent,  so  as  to  allow  move- 
ments of  the  neck.  The  two  upper  members  of  the  cervical 
series  are  much  modified  in  shape,  in  order  to  permit  movements 
of  the  head  on  the  vertebral  column. 

The  first  vertebra  which  articulates  with  the  occipital  bone 
of  the  skull  is  called  the  atlas^  and  the  second  vertebra,  called 
the  axis^  articulates  above  with  the  atlas,  and  below  with  the 
third  cervical  vertebra. 

The  shape  of  these  tv\^o  bones,  and  the  manner  in  which 


22 


Elementary  Physiology. 


they  are  articulated  together,  is  shown  in  the  accompanying 


drawings 


The  atlas  is  a  ring  of  bone,  the  body  having  disappeared, 
and  where  the  body  would  be  if  present  the  odo?itoid  process 


ODONTOID      PROCESS 


NF,   ARTIC.PROC. 


Fig.  io. — Axis,  from  the  right  side.     (Drawn  by  D.  Gunn.) 

of  the  axis  projects  through,  round  which,  as  on  a  pivot,  the 
atlas  carrying  the  head  can  turn.    The  strong  transverse  ligmnent 
passing  behind  the  odontoid  process  completes  the  socket. 
On  the  upper  surface  of  the  atlas  are  two  articular  facets 


ODONTOID      PSOCESS 


IMF    ARTIC     PROCESS 


SUP  ARTIC    PROCESr 


rRANSV.  PHOC. 


ITRANSV.  PROO. 
INF.  ARTIC.  PROCESS     ; 

Fig.  II. — Atlas  and  axis,  from  before.     (Drawn  by  D.  Gunn.) 

of  the  form  shown  in  the  drawing  (Fig.  9).  These  are 
slightly  concave  on  their  surface,  and  receive  two  similarly 
shaped  but  convex-surfaced  facets  placed  on  the  occipital  bone 
of  the  skull  (see  Fig.  13),  and  called  the  occipital  condyles.     It 


TJie  Skeleton  and  its  Articulations.  23 

is  by  a  rocking  movement  of  the  condyles  on  tiiese  articular 
facets  of  the  atlas  that  nodding  of  the  head  up  and  down  is 
effected. 

The  varied  movements  of  the  head  on  the  neck  are  mix- 
tures of  these  two  movements  simultaneously — namely,  of  an 
up-and-down  movement  brought  about  by  the  rocking  of  the 
occipital  condyles  on  the  atlas,  and  of  a  rotation  movement  of 
the  atlas  on  the  axis  around  the  odontoid  process.  Changes 
in  position  of  the  head  are  further  assisted  by  movements  of 
the  cervical  vertebras,  i.e.  by  bending  of  the  neck.  If  the  head 
be  turned  round  sharply  to  one  side,  and  the  neck  be  felt  on  the 
other  side,  a  hard  mass  will  be  felt  passing  from  behind  the  ear 
down  to  the  breast-bone  and  collar-bone  on  the  front  of  the  chest. 
This  is  the  mass  of  the  sterno-mastoid  muscle.  When  the 
muscle  of  one  side  is  contracted,  and  not  the  opposite  one,  the 
head  is  rotated  towards  the  side  on  which  the  muscle  lies,  and  de- 
pressed on  that  side.  When  both  sterno-mastoids  are  contracted 
the  head  is  depressed  in  front.  The  head  is  elevated  by  the 
muscles  of  the  back  of  the  neck  acting  in  opposition  to  the 
muscle  (sterno-mastoid)  just  referred  to,  which  has  been  cited 
because  it  lies  superficially  and  can  easily  be  felt.  Besides 
these  there  are  other  muscles  which  combine  in  producing  the 
movements  of  the  head.  When  a  movement  takes  place,  nerve 
impulses  are  sent  to  these  various  muscles  along  the  nerves 
belonging  to  them,  and  a  nicely  adjusted  amount  of  stimulus 
is  given  to  each  muscle,  so  as  to  cause  just  the  proper  amount 
of  contraction  in  each  in  order  to  produce  the  desired  degree 
of  motion  when  combined  with  the  action  of  the  other  muscles 
involved. 

Beneath  the  seven  cervical  vertebrae  come  the  twelve  dorsal 
or  ihoradc  vertebrae.  These  are  more  strongly  built  than  the 
cervical,  as  is  fitting,  since  they  have  to  support  a  much  greater 
load.  The  cervical  vertebrae  carry  only  the  weight  of  the  head  j 
but  the  dorsal  have  the  ribs  attached  along  their  sides,  and 
have  communicated  to  them  by  these  the  weight  of  the  upper 
part  of  the  trunk  {i.e.  the  thorax)  and  of  the  upper  limbs.  The 
dorsal  vertebrae  may  be  distinguished  by  the  facets  for  the  ribs 
on  their  bodies  and  transverse  processes. 


24  Elermntary  Physiology, 

The  next  five  vertebrse  are  the  hmibar  (or  loin)  vertebrae. 
These  are  very  massive  in  their  structure,  because  they  have  to 
support  the  whole  weight  of  the  part  of  the  body  above  them, 
and  transmit  it  to  the  hip-bones.  They  have  no  ribs  attached 
to  them,  and  the  soft  parts  (abdominal  viscera)  in  the  part  of 
the  trunk  opposite  to  them  are  only  protected  by  the  strong 
sheets  of  muscle  passing  from  the  upper  brim  of  the  pelvis  to 
the  lower  ribs. 

The  lowest  of  the  lumbar  vertebrae  is  seated  on  a  strong 
bone  called  the  sacrum^  shaped  like  a  curved  wedge  (see  Figs. 
5  and  6),  and  formed  by  the  fusion  together  of  a  number  of 
imperfect  vertebrse.  The  vertebrse  which  make  up  the  sacrum 
are  usually  five  in  number,  and  early  in  life  exist  as  distinct 
bones,  but  later  become  completely  fused  together  into  one 
bone.  By  two  large  articular  surfaces  {atiricular  surfaces),  one 
on  each  side,  the  sacrum  articulates  with  the  two  hip-bones,  and 
lying  between  these  forms  a  portion  of  the  pelvis  (see  Fig.  19). 
Strong  bands  and  sheets  of  ligament  pass  from  the  sacrum  to 
the  hip-bones,  and  a  great  part  of  the  weight  is  borne  by  these 
ligaments ;  so  that  the  vertebral  column  and  its  load  are  in  part 
borne  by  the  articulation  with  the  hip-bones,  and  in  part  are 
borne  hammock-fashion  by  the  ligaments  passing  from  hip- 
bones to  sacrum.  The  coccyx  is  attached  to  the  lower  end  of 
the  sacrum ;  it  consists  of  from  three  to  five  (usually  four) 
rudimentary  vertebrae,  which  are  commonly  fused  together  into 
one  bone  (see  Figs.  5  and  6). 

It  will  be  seen  from  the  above  description  that  the  vertebral 
column  is  a  strong  and  somewhat  flexible  pillar  of  bones,  which 
upholds  the  weight  of  the  part  of  the  body  lying  above  the 
hips,  and  affords  an  attachment  to  the  ribs ;  besides  this,  it 
furnishes  a  long  cavity  in  which  the  spinal  cord  lies.  At  the 
upper  end  this  spinal  cord  enlarges  into  the  brain,  which  is 
lodged  in  the  large  cavity  of  the  cranium,  occupying  the 
greater  part  of  the  volume  of  the  skull. 

The  Skull. 

The  skull  may,  for  purposes  of  description,  be  considered 
in  two  parts — viz.  the  cranium,  or  brain-case,  and  the  face. 


The  Skeleton  and  its  Articulations. 


25 


The  cranium  occupies  the  upper  and  back  part  of  the  skull, 
and  is  formed  by  eight  bones;  the  face  forms  the  front  and 
lower  part,  and  is  made  up  of  fourteen  bones,  making  twenty- 
two  bones  in  the  skull  in  all.  The  position  and  names  of  the 
various  skull-bones  are  shown  in  the  accompanying  figures. 


Fig.  12.— Lateral  view  of  the  skull.     (Allen  Thomson.)    \ 

frontal  bone  ;  2,  parietal  bone  at  the  upper  temporal  line  ;  X  X,  coronal  suture  ;  3,  on 
the  occipital  bone  ;  3',  external  occipital  protuberance  ;  4,  sphenoid  bone  ;  5,  squamous 
part  of  temporal ;  6,  the  same  at  the  root  of  the  zygoma,  immediately  over  the 
external  auditory  meatus  ;  7,  mastoid  portion  of  temporal,  at  the  front  of  which  is 
the  mastoid  process  ;  8,  left  condyle  of  occipital  bone  ;  9,  anterior  nasal  aperture  ; 
ID,  on  the  lachrymal  bone  in  the  inner  wall  of  the  orbit  ;  11,  malar  bone,  near  its 
junction  with  the  zygoma  ;  12,  superior  maxilliarybone  ;  13,  ramus  of  the  lower  jaw  ; 
14,  body  of  the  lower  jaw,  near  the  mental  foramen. 


The  bones  of  the  skull,  with  the  exception  of  the  lower  jaw, 
are  immovably  united  together  where  they  come  into  contact 
by  the  form  of  articulation  known  as  a  sutwe.     These  sutures 


26 


Elementary  Physiology. 


are  often  dentated  or  serrated  in  outline  (see  Fig.  12);  the 
serrations  strengthen  the  junction  between  the  two  bones. 
When  it    is   desired  to  study  the  shape   of    the   skull-bones 


Fig.  13.— External  base  of  the  skull.  (Allen  Thomson.)  \ 
I  palate  plate  of  the  superior  maxillary  bone  ;  2,  palate  plate  of  the  palate  bone  ; 
7  vomer  bone  ;  12,  jugular  foramen  ;  13,  articular  emmence  of  the  temporal  bone  ; 
14,  external  auditory  meatus  ;  15,  glenoid  fossa  ;  18,  basilar  process  of  the  occipital 
bone ;  19,  condyle  of  the  occipital  bone  ;  20,  is  placed  in  the  foramen  magnum  , 
23,  external  occipital  crest  running  down  from  the  protuberance  ;  24,  superior  curved 
line  of  the  occipital  bone  ;  25,  26,  inferior  curved  line. 

separately,  the  skull  must  be  disarticulated  or  separated  into  its 
constituent  bones;  but  it  is  impossible  to  describe  here  in 
detail  all  these  various  bones/ 

'  For  such  a  description,  see  Quain's  "Anatomy,"  vol.  ii.  pt.  i. 


The  Skeleton  and  its  Articulations. 


27 


The  cranial  bones,  by  their  union,  form  a  hollow  case  of 
bone  for  the  protection  of  the  brain ;  the  cavity  so  formed  is 
rounded  or  spheroidal  above  but  is  flatter  at  its  base.  The  base 
is  further  divided  by  ridges  of  bone  into  three  hollows,  or  fossae, 


21-/-^ 


Fig.   14.— Section  of  the  adult  skull  a  little  to  the  left  of  the  median  plane. 
(Allen  Thomson.)    i. 

I,  nasal  bone  ;  2,  perpendicular  plate  of  the  ethaioid  bone  with  olfactory  foramina  and 
grooves  at  its  upper  part  ;  3,  vomer ;  6,  inner  surface  of  the  frontal  bone  ;  7,  of  the 
parietal  bone  ;  8,  squamous  part  of  the  temporal  bone  ;  9,  on  the  occipital  bone  below 
the  internal  occipital  protuberance  ;  10,  external  occipital  protuberance  ;  17,  anterior 
nasal  spine  ;  19,  on  the  inner  surface  of  the  ramus  of  the  lower  jaw,  below  the 
sigmoid  notch,  and  above  t'le  inferior  dental  foramen. 

which  correspond  to  divisions  of  the  brain ;  the  posterior  fossa 
lodges  the  cerebellum.,  or  lesser  brain  ;  the  middle  and  anterior 
fossae  conform  to  the  shape  of  the  base  of  the  greater  brain,  or 
cerebnim.  Besides  smaller  holes,  or  foramina,  for  the  passage 
of  nerves  and  blood-vessels  to  and  from  the  brain,  the  base  of 


28  Elementary  Physiology. 

the  cranium  contains  a  larger  opening,  the  foramen  magnum 
(see  Fig.  13),  through  which  the  portion  of  the  central  nervous 
system  called  the  medulla  oblongata  passes,  uniting  brain  and 
cord.  The  bones  of  the  face  form  the  greater  parts  of  the 
orbits  for  the  accommodation  of  the  eyes,  the  nose,  the  hard 
palate,  the  upper  and  lower  jaws.  The  outline  of  the  nose  is 
completed  by  the  nasal  cartilages.  The  nasal  bones  and  the 
cartilages  completing  the  nose  may  be  felt  through  the  skin 
during  life ;  the  flexible  front  portion  is  cartilage,  the  fixed  part 
behind  is  formed  by  the  nasal  bones.  The  hard  palate,  or 
roof  of  the  mouth,  is  formed  by  the  superior  maxillary  bones, 
which  also  bear  the  teeth  of  the  upper  jaw.  It  is  continued 
back,  in  life,  by  the  fleshy  velum.,  or  soft  palate.,  behind  which 
the  posterior  7iares,  or  inner  openings  of  the  nostrils,  com- 
municate with  the  pharynx.^  The  prominences  of  the  cheek 
are  formed  by  the  malar  bones. 

The  lower  jaw  {inferior  maxil-a)  is  the  only  bone  in  the 
skull  which  has  a  movable  articulation.  A  section  through 
this  joint  has  already  been  given  as  an  example.  We  may 
now  consider  somewhat  more  fully  the  movements  which  take 
place  at  this  joint.  The  lower  jaw  is  capable  of  movement 
both  upwards  and  downwards,  backwards  and  forwards,  and 
from  side  to  side.  These  movements  are  necessary  to  give  a 
grinding  action  between  the  teeth. 

The  movements  of  the  jaw  are  brought  about  by  several 
muscles ;  the  strongest  set  are  those  which  raise  it  and  bring  it 
into  contact  with  the  upper  jaw,  against  the  resistance  of 
anything  which  may  be  interposed.  The  jaw  is  depressed  by  its 
own  weight,  but  may  be  forcibly  lowered  by  the  action  of 
special  muscles,  the  chief  of  which  are  the  digastrics,  attached 
beneath  the  jaw  in  front  and  running  backward  parallel  to  the 
line  of  the  jaw  on  each  side  (see  12,  Fig.  15). 

When  the  jaw  opens  the  condyle  and  interarticular  cartilage 
are  pulled  forward,  as  you  may  find  by  applying  your  forefinger 
to  the  articulation  just  in  front  of  the  external  opening  of  the 

'  The  pharynx  is  the  upper  funnel-shaped  part  of  the  alimentary  canal 
which  leads  to  the  gullet,  or  oesophagus  ;  into  it  the  mouth  and  the  windpipe, 
or  trachea,  also  open. 


Fig.  15. — Deep  muscles  of  the  left  side  of  the  head  and  neck.     (^Allen  Thomson, 
after  Bourgery.)    ^] 

vertex  of  head  ;  /',  superior  curved  line  of  occipital  bone  ;  c,  ramus  of  lower  jaw  ;  c',  its 
coronoid  process  ;  d,  hyoid  bone ;  e,  sternal  end  of  clavicle  ;  e',  acromial  end  ;  _/j 
malar  bone  divided  to  show  the  insertion  of  the  temporal  muscle ;  y',  divided 
zygoma,  and  external  ligament  of  the  jaw  ;  £;  thjToid  cartilage  ;  /i,  placed  on  the 
lobule  of  the  auricle,  points  to  the  styloid  process  ;  i,  temporal  muscle  ;  2,  corni- 
gator  supercilii  ;  3,  pyramidaiis  nasi  ;  4,  compressor  naris  ;  5,  levator  labii  superioris  ; 
6,  levator  anguli  oris  ;  7,  outer  part  of  the  orbicularis  oris,  the  part  below  the  nose 
has  been  removed ;  8,  depressor  alae  nasi ;  9,  points  to  the  buccinator  muscle, 
through  which  the  parotid  duct  is  seen  passing ;  10,  depressor  labii  inferioris ; 
II,  levator  menti ;  12,  12,  anterior  and  posterior  bellies  of  the  digastric;  13,  stylo- 
hyoid muscle  ;  14,  mylo-hyoid  ;  15,  hyoglossus,  between  which  and  13  is  seen  a 
part  of  the  stylo-giossus  ;  16,  sterno-hyoid  ;  17,  on  the  cla\dcle,  indicates  the  posterior, 
and  17',  the  anterior  belly  of  the  omo-hyoid  ;  18,  sterno-thyroid  ;  19,  thyro-hyoid  ; 
20,  21,  on  the  sterno-mastoid  muscle,  point,  the  first  to  the  middle,  the  second  to 
the  lower  constrictor  of  the  pharyn.x  ;  22,  trapezius  ;  23,  upper  part  of  the  complexus  ; 
24,  25,  splenius;  26,  levator  angulae  scapulae;  27,  middle  scalenus;  +,  posterior 
scalenus  ;  28,  anterior  scalenus. 


30  Elementary  Physiology. 

ear;  the  same  thing  happens  when  the  lower  jaw  is  forcibly 
moved  forward.  If  one  forefinger  be  placed  in  this  position 
on  either  side,  and  the  jaw  be  then  moved  from  side  to  side 
laterally,  it  will  be  found  that  one  condyle  moves  forward  while 
the  other  remains  in  the  groove.  These  movements  are 
brought  about  by  the  action  of  the  external  pterygoid  muscles, 
shown  ill  the  drawing  (Fig.  i6).  When  these  muscles  contract 
together  On  the  two  sides,  the  jaw  is  drawn  forward ;  when  they 
contract  alternately,  the  jaw  is  moved  from  side  to  side. 


Fig.  i6. — The  pterygoid  muscles  from  outside.     (G.  D.  T.)    \ 

The  masseter  muscle,  the  greater  portion  of  the  zygomatic  arch,  the  temporal  muscle 
with  the  coronoid  process,  and  a  large  part  of  the  ramus  of  the  jaw  have  been 
removed,  i,  external  pterygoid  ;  the  figure  is  placed  on  the  lower  head ;  2,  internal 
pterygoid. 

The  strong  set  of  muscles  which  raise  the  lower  jaw  com- 
prise, on  each  side,  the  internal  pterygoids  (2,  Fig.  16)  on  the 
inner  side  of  the  jaw,  the  masseters  outside,  and  the  temporals 
above  (see  i,  Fig.  16).  The  action  of  the  temporal  and 
masseter  muscles  may  be  felt  through  the  skin.  The  temporal, 
above  and  in  front  of  the  ear  (on  the  temple),  may  be  felt  to 
contract  by  its  swelling  up  as  the  jaw  is  moved  upwards  from 
an  open  position;  the  masseter  may  be  similarly  felt  at  the 
angle  of  the  jaw,  if  the  teeth  are  ground  together  after  the  jaw 
is  closed. 

Each  jaw  is  armed  with  a  number  of  teeth,  for  the  purpose 


The  Skeleton  and  its  Ai'ticulations. 


31 


of  grinding  or  masticating  the  food.  The  full  number  of  teeth 
in  the  adult  is  sixteen  in  each  jaw.  The  teeth  are  symmetrical 
in  character  in  each  jaw,  and  in  each  side  of  each  jaw.  Of 
the  eight  in  each  half  of  each  jaw,  the  two  nearest  the  front 
(front  teeth)  are  called  incisors^  or  cutting  teeth ;  the  next  is  the 
canine  tooth,  which  is  longer  than  the  others,  and  is  used  for 
piercing  hard  food;  the  remaining  five  are  grinding  teeth,  and  are 
more  or  less  flat-topped,  but  provided  with  cusps  or  eminences, 
so  as  to  give  an  uneven  surface  serviceable  for  grinding.  The 
two  nearer  the  front  of  these  five  are  transitional  in  character, 
and  are  called  bictispids  ;  the  other  three  are  the  molar,  or  "  true 
molar  "  teeth.  During  mastication  the  food  is  kept  under  the 
teeth  by  the  action  of  the 
muscles  of  the  cheeks  and 
tongue,  which  return  it  from 
either  side  as  it  becomes 
displaced  by  the  previous 
action  of  the  jaw,  so  replac- 
ing it  between  the  teeth. 


The  Thorax. 

The  bony  framework  of 
the  chest,  or  thorax,  may 
be  regarded  as  a  kind  of 
open  basketwork  of  bone, 
which  gives  stability  and 
elasticity  to  the  outline  of 
this  upper  compartment  of 
the  trunk,  and  at  the  same  i, 
time,  when  acted  upon  by 
appropriate  muscles,  allows 
the  volume  of  the  chest 
cavity  to  be  rhythmically 
altered  in  the  act  of  breath- 
ing, and  so  occasions  air  to 
which  occupy  the  greater  part 

The  bony  thorax,  as  show 


Fig.  17. — Front  view  of  the  thorax. 

manubrium  ;  2,  is  close  to  the  place  of  union 
of  the  first  costal  cartilage ;  3,  clavicular 
notch  ;  4,  body  of  the  sternum  ;  5,  ensiform 
process  ;  6,  groove  on  the  lower  border  of 
the  ribs  ;  7,  the  vertebral  end  of  the  ribs ; 
8,  neck  ;  g,  tuberosity  ;  10,  costal  carti- 
lage ;  12,  first  rib ;  13,  its  tuberosity  ;  14, 
first  dorsal  vertebra;  15,  eleventh  rib;  16, 
twelfth  rib. 


pass  into  and  out  of  the  lungs, 

of  the  space  inside. 

n  in  the  woodcut,  is  formed  by 


32  Elementary  Physiology. 

the  twelve  thoracic  vertebrae,  and  by  the  twelve  ribs  on  each 
side  (twenty-four  in  all),  and  is  completed  in  front  by  the  rib 
cartilages,  which  are  attached  to  the  sternum,  or  breast-bone, 
lying  in  the  mid  line  in  front. 

The  thorax  is  shaped  like  a  rounded  and  truncated  cone, 
and  is  much  longer  behind  than  in  front.  The  ribs  at  first  slope 
backwards  from  their  attachments  to  the  vertebrse,  and  thus 
give  rise  to  the  hollow  in  the  middle  line  of  the  back ;  they 
then  curve  round  forward  in  the  rest  of  their  length,  so  com- 
pleting the  posterior  and  lateral  wall  of  the  thorax.  In  this 
latter  part  of  their  course  the  ribs  slope  downwards.  In  front 
the  ribs  are  articulated  with  the  costal  cartilages,  which  unite 
them  to  the  sternum,  in  the  manner  shown  in  the  woodcut. 

This  bony  framework  is  in  the  body  converted  into  a  closed 
box  by  soft  tissues.  Two  sheets  of  muscles  called  the  intercostals 
(see  Fig.  i8)  lie  between  the  successive  ribs,  their  fibres  passing 
obliquely  from  the  one  rib  to  the  other.  The  floor  of  the 
cavity  of  the  thorax  is  formed  by  a  large  sheet  of  muscle  and 
tendon,  called  the  diaphragm^  which  is  attached  all  round  the 
lower  border  of  the  thorax,  and  has  a  strong  central  tendon. 
The  diaphragm  completely  shuts  off  the  upper  cavity  of  the 
trunk  {i.e.  the  thorax)  from  the  lower  cavity  or  abdomen.  It 
is  pierced  at  its  centre  and  posterior  part  by  the  tubes  and 
vessels  which  must  pass  from  one  cavity  to  another.  These 
are  the  great  blood-vessels  carrying  the  blood  to  and  from  the 
lower  part  of  the  body,  and  the  oesophagtis,  or  tube  which 
conveys  the  food  to  the  stomach. 

It  must  be  remembered  that  the  ribs  are  free  to  move  to  a 
certain  extent  up  and  down,  around  their  attachments  to  the 
vertebrae  as  an  axis.  Since  the  ribs  slope  downward,  anything 
which  raises  them  must  make  them  stand  out  more  at  right 
andes  to  the  vertebral  column,  and  hence  must  increase  the 
capacity  of  the  chest  by  increasing  its  girth.  The  increase 
takes  place  both  from  before  backward  and  laterally.  The 
raising  of  the  ribs  takes  place  through  the  contraction  of  those 
layers  of  intercostal  muscles  which  are  known  as  exte7'nal^ 
because  they  lie  nearer  the  skin  than  the  other  layer,  which  are 
called  the  mteimal  intercostals.     The  external  intercostals  are 


The  Skeleton  and  its  Artiailations. 


33 


shown  in  the  accompanying  figure  in  the  upper  intercostal 
space.  In  the  lower  space,  the  external  intercostals  have  been 
represented  as  removed,  so  as  to  show  the  internal  intercostals. 
A  glance  at  the  direction  of  the  muscle  fibres  of  the  external 
intercostals  in  the  figure  will  show  that  when  these  fibres 
shorten,  the  ribs  to  which  they  are  attached  must  together 


Fig.  rS.— Intercostal  muscles  of  the  fifth  and  sixth  spaces.     (Allen  Thomson,  after 

Cloquet.)    \ 

A,  from  the  side  ;  B,  from  behind. 
IV.,  fourth  dorsal  vertebra  ;  V,  V,  fifth  rib  and  cartilage ;  i,  i,  levatores^  costarum 
muscles,  short  and  long  ;  2,  2,  external  intercostal  muscle  ;  3,  3,  internal  intercostal 
layer,  shown  in  the  lower  space  by  the  removal  of  the  external  laj'er,  and  seen  in  A 
in  the  upper  space,  in  front  of  the  external  layer ;  the  deficiency  of  the  internal  layer 
towards  the  vertebral  column  is  shown  in  B. 


move  upwards.^  The  action  of  the  internal  intercostals,  the 
fibres  of  which  slant  in  the  opposite  direction  in  forced  expira- 
tion, is  to  lower  the  ribs.  In  ordinary  breathing  the  weight  of 
the  chest  is,  however,  sufficient.     The  internal  intercostals  are 

^  The  first  rib  is  held  fixed  by  the  muscles  above  it ;   and  hence  when 
the  external  intercostals  contract  movement  upwards  must  take  place. 

D 


34  Elementary  Physiology. 

continued  between  the  rib  cartilages  in  front,  and  as  these  slope 
tLpwards  this  portion  of  the  internal  intercostals  will  tend  to 
raise  the  cartilages  and  so  aid  in  inspiration. 

The  action  of  the  ribs  is  not  the  only  means  by  which  the 
capacity  of  the  thorax  is  altered ;  it  has  in  the  diaphragm  a 
movable  muscular  floor.  The  diaphragm  is  not  a  sheet  lying  all 
in  one  plane  (see  Fig.  52) ;  it  is  somewhat  dome-shaped,  the  top 
of  the  dome  being  turned  towards  the  thorax,  and  hence,  when 
the  muscular  sheet  which  forms  its  peripheral  part  contracts,  the 
dome  is  drawn  down  towards  the  abdomen,  thus  increasing 
the  capacity  of  the  thorax  from  top  to  bottom.  Thus,  if  at  the 
same  time  the  ribs  are  raised  and  the  diaphragm  contracted, 
the  dimensions  of  the  thorax  are  increased  in  all  directions. 

Breathing  by  the  action  of  the  ribs  is  spoken  of  as  costal 
respiration,  and  breathing  by  the  action  of  the  diaphragm  as 
diaphragmatic  or  abdo7ninal  respiration. 

The  increase  and  diminution  in  the  volume  of  the  thorax 
causes  a  quantity  of  air  alternately  to  enter  and  leave  the  lungs, 
which  fill  the  greater  part  of  the  thoracic  cavity ;  for  the 
increase  in  volume  causes  a  suction  on  the  contents  of  the 
thoracic  cavity.  Now,  the  lungs  are  the  only  organs  within 
the  thorax  which  can  expand  to  the  necessary  extent  and 
prevent  the  formation  of  a  vacuous  space  inside.  The  lungs 
can  expand  in  this  manner  because  they  consist  essentially  of 
air-chambers  with  elastic  walls,  to  which  the  ultimate  sub- 
divisions of  the  windpipe,  or  trachea,  lead.  When  the  thorax 
increases  in  volume,  the  atmospheric  pressure  blows  the  air 
down  the  trachea  and  distends  these  air-chambers  so  as  to  fill 
the  increased  space,  thus  increasing  the  volume  of  the  lungs 
and  bringing  a  supply  of  air  to  them.  On  the  contrary,  when 
the  ribs  fall  and  the  diaphragm  rises,  the  capacity  of  the  chest 
diminishes,  air  is  forced  out,  and  the  air-spaces  in  the  lungs 
become  less  distended.  The  purpose  of  this  respiration  or 
alternate  sucking  of  air  in  {inspiration)  and  blowing  it  out 
{expiration)  we  shall  learn  subsequently. 


The  Skeleton  and  its  Artiadations. 


r:> 


The  Pelvis. 

The  bony  pelvis  (see  Fig.  19)  is  a  strong  ring  of  bone 
formed  by  the  union  of  the  two  hip-bones  with  the  sacrum  j 
this  receives  the  weight  of  the  upper  part  of  the  body  and 
transmits  it  to  the  thigh-bones.  The  thigh-bones  articulate 
with  the  hip-bones  in  two  deep  cups,  one  on  each  bone,  called 
the  acetabula  (see  Figs.  19  and  20). 


Fig.  19. 


-Adult  male  pelvis  seen  from  before,  in  the  erect  attitude  of  the  body. 
(Allen  Thomson.)     \ 


I,  2,  anterior  c.Ktremities  of  the  iliac  crests  ;  3,  4,  acetabula ;  5,  5,  thyroid  foramina  ; 
6,  subpubic  angle  or  arch. 

When  the  ligaments  and  muscles  attached  to  the  pelvis  are 
present,  it  forms  a  basin-shaped  cavity,  the  floor  of  which 
supports  to  a  certain  extent  the  contents  of  the  abdominal 
cavity  {abdominal  viscera). 

Each  hip-bone  {os  innominaticni)  is  originally  formed  from 
three  distinct  bones,  which  persist  in  youth  but  become  fused 
together  in  the  adult  (eighteenth  to  twentieth  year).  The  three 
bones  are  named  iliiun,  ischium,  and  p^dns  respectively  ;  the 
ilium  is  the  upper  and  larger  part  of  the  bone  which  articulates 
with  the  sacrum  and  extends  down  to  form  part  of  the 
acetabulum ;  the  pubis  lies  in  front,  and  unites  with  its  fellow 
to   form  an  arch   {tJie  pubic  arch) ;    the  ischium  is   the  lower 


36  Elementary  Physiology. 

portion  of  the  bone,  that  on  which  one  rests  in  the  sitting 
posture.     These  names  for  the  different  parts  of  the  bone  are 


Fig.  20.— Transverse  oblique  section  of  the  pelvis  and  hip-joint,  cutting  the  first  sacral 
vertebra  and  the  symphysis  pubis  in  their  middle,  from  a  male  subject  of  about  nine- 
teen years  of  age.     (Allen  Thomson.)    \ 

I,  first  sacral  vertebra  ;  2,  ilium  ;  3,  posterior  sacro-iliac  ligament ;  6,  small  sacrosciatic 
ligament ;  7,  great  sacro-sciatic  ligament ;  8,  placed  in  front  of  the  symphysis  pubis, 
in  the  cut  surface  of  which  the  small  median  cavity,  the  adjacent  cartilaginous 
plates,  and  the  anterior  and  posterior  ligamentous  fibres  are  shown  ;  10,  cartilaginous 
surface  of  the  cotyloid  cavity,  through  the  middle  of  which  the  incision  passes  trans- 
versely, dividing  the  interarticular  ligament  and  the  fat  in  the  fossa  acetabuli ; 
II,  cotyloid  ligament ;  12,  interarticular  ligament  connected  with  the  transverse  part 
of  the  cotyloid  ligament ;  13,  placed  on  the  cut  surface  of  the  head  of  the  left  femur 
near  the  depression  where  the  interarticular  ligament  is  attached  ;  14,  14  ,  upper  and 
lower  parts  of  the  capsular  ligament. 

still  used  even  after  union  into  one  bone  has  taken  place,  in 
order  to  describe  the  different  regions  of  the  bone.  The  upper 
and  posterior  brim  of  the  pelvis  is  known  as  the  iliac  crest ;  it 


The  Skeleton  and  its  Arliailations.  37 

forms  in  the  body  the  eminence  of  the  haunches.  To  the 
back  of  the  ilium  are  attached  the  strong  hip  muscles  {glutei 
muscles)  which  straighten  the  leg  on  the  body  at  the  hip  joint ; 
they  are  used,  for  example,  in  straightening  the  body  at  the 
hips  when  rising  from  a  stooping  posture,  as  also  in  walking 
and  in  running.  The  muscles  which  flex  the  leg  on  the  trunk 
at  the  hip  joint  lie  in  front. 

The  sacrum  and  hip-bones  are  immovably  articulated 
together.  Where  the  two  hip-bones  meet  in  front  a  pad  of 
cartilage  is  interposed  between  them,  which  serves  the  same 
purpose  as  the  intervertebral  discs  in  the  case  of  the  vertebrae. 
This  junction  of  the  two  bones  is  called  the  pubic  symphysis. 

The  articulation  of  the  thigh-bone  with  the  hip-bone  is  one 
of  the  best  examples  in  the  body  of  what  is  known  as  a  ball- 
and-socket  joint.  A  similar  kind  of  joint  is  found  in  the 
shoulder,  where  the  humerus  (the  bone  of  the  upper  arm)  is 
joined  to  the  scapula  (or  shoulder-blade) ;  but  here  the  socket 
is  not  so  deep,  and  the  degree  of  movement  is  hence  consider- 
ably greater.  Nevertheless  the  amount  of  free  movement  in 
every  possible  direction  at  the  hip  joint  is  very  large. 

The  nature  of  the  ball-and-socket  joint  is  well  seen  in  the 
accompanying  drawing  (Fig.  20),  showing  a  section  through  the 
hip  joint.  The  round  ball-shaped  head  of  the  femur  (thigh- 
bone) is  covered  with  smooth  articular  cartilage ;  as  is  also  the 
opposed  cavity  of  the  acetabulum,  except  at  its  bottom  where 
it  does  not  touch.  The  joint  is  strengthened,  in  this  case,  by 
a  strong  ligament  uniting  the  middle  of  the  head  of  the  femur 
to  the  bottom  of  the  socket  \  this  is  called  the  inferartic^Uar 
ligament.  There  is  as  usual  a  capsule  surrounding  the  joint 
into  which  the  lubricating  synovial  fluid  is  secreted.  The 
joint  is  further  strengthened  by  strong  ligamentous  bands 
which  pass  over  the  capsule  from,  hip-bone  to  thigh-bone. 

The  Limb  Bones. 

The  bones  of  the  upper  and  lower  limb  exhibit  a  marked 
amount  of  similarity,  or  homology^  in  their  arrangement.  The 
shoulder  joint  corresponds  to  the  hip  joint;   the  humerus,  or 


38  Elementary  Physiology, 

arm-bone,  articulating  with  the  shoulder-blade,  which  although 
so  different  in  shape  corresponds  to  the  hip-bone.  In  the 
upper  arm,  as  in  the  thigh,  there  is  but  one  bone ;  while  in  the 
forearm  there  are  two  bones,  the  ulna  and  radius,  correspond- 
ing respectively  to  the  tibia  and  fibula  in  the  leg.  Next  there 
are  a  number  of  small  bones  of  the  wrist  (carpal  bones), 
corresponding  to  a  number  of  small  bones  (tarsal  bones)  in 
the  ankle.  More  distal  to  these  there  are  a  row  of  bones,  five 
in  each  case,  called  n^etacarpals  in  the  hand  and  7netatarsals  in 
the  foot.  Next  are  three  rows  of  bones,  in  each  case,  form- 
ing the  fingers  and  toes  respectively ;  these  bones  are  called 
phalanges  (sing,  phalanx)  both  in  the  hand  and  foot.  In  the 
case  of  the  thumb  and  great  toe  there  are  but  two  phalanges ; 
the  other  digits  possess  three  each. 

The  Upper  Limb. 

The  upper  limb   is  attached  to  the  body  by  the  shoulder- 
girdle,  which  consists  of  two  bones — the  clavicle^  or  collar-bone, 

and    the    scapula^    or 
^^!^^^^W^        shoulder-blade.     The 

OE.LTOID  yW^^^^^^^^S^m  1-1  1 

J"B     x<^^^ttiPW"'|^W^a        clavicle,  as   shown   m 


ACROMIAL  TRAPEZOID 
FACET 


RIAL  Pci^R^SIt.^'"'^   /        the  drawmg,  is  a  bone 

STERNAL  •.1  111 

FACET     With    a    double    curve 


iROOVE    OF    SUBCLAVIUS 

somewhat  like  an  italic 
^  .       ,    .  f.     It  may  be  felt  be- 

FiG.  21.— Right  clavicle,  from  below.     (Drawn  by  i       -i  i  • 

T.  w.  P.  Lawrence.)  ncath  the  skin  at  the 

junction   of  neck  and 

thorax  in  front.     It  is  articulated  to  the  breast-bone  in  front, 

and  passes  outward   and   backward   to  articulate  at  its  other 

outer  end  with  a  process  of  the  scapula  called  the  acroinion 

process  (see  Figs.  22  and  23). 

The  scapula  is  a  triangular-shaped  plate  of  bone  which  is 
placed  at  the  back  of  the  shoulder,  the  shortest  side  of  the 
triangle  being  directed  upwards.  Its  anterior  surface  is  some- 
what concave,  following  the  shape  of  the  underlying  parts  of 
the  body,  and  its  posterior  surface  is  correspondingly  convex. 
Across  its  dorsal  surface  there  runs  a  ridge  of  bone  called  the 


The  Skeleton  and  its  Artiadations. 


39 


spine  of  the  scapula,  which  becomes  more  prominent  as  it 
passes  outwards  and  ends  in  the  acromion  process  above 
referred  to,  where  the  end  of  the  clavicle  is  attached. 

Beneath  the  acromion  process  is  the  head  of  the  scapula, 
which  replaces  the  outer  angle  of  the  triangle  by  a  rounded 
or    elliptical  -  shaped    arti  - 


ACROMION 


cular  surface  (the  glenoid 
fossa)  covered  with  cartilage, 
against  which  the  head  of 
the  humerus^  or  upper  arm- 
bone,  moves.  The  head  of 
the  humerus  has  a  spherical 
surface,  also  coated  with 
cartilage,  and  the  joint  is 
enclosed  by  a  capsule,  so  as 
to  make  a  shallow  ball-and- 
socket  joint.  The  scapula 
and  its  processes,  together 
with  the  clavicle,  give  at- 
tachment to  the  muscles 
which  move  the  arm  at  the 
shoulder-joint. 

The  attachments  of  the 
sternum,  shoulder-girdle,  and  humerus  are  represented  in 
Fig.  23 ;  it  may  be  pointed  out  that  there  is  a  disc  of  cartilage 
at  the  articulation  between  the  clavicle  and  sternum. 

The  humeims^  or  bone  of  the  upper  arm  (see  Fig.  24), 
extends  from  the  shoulder  to  the  elbow ;  it  articulates  at  the 
shoulder  with  the  scapula,  and  at  the  elbow  with  the  ulna  and 
radi^Ls^  the  bones  of  the  forearm.  The  upper  extremity  of  the 
humerus  includes  the  head,  a  hemispherical  articular  surface 
which,  as  above  described,  forms  part  of  the  shoulder-joint; 
the  neck,  a  narrow  groove  marking  off  the  head  from  the  rest 
of  the  bone;  and  the  great  and  small  tuberosities,  which  are 
eminences  giving  attachment  to  certain  of  the  shoulder  muscles. 
The  shaft  or  body  of  the  bone  bears  rough  patches  on  its  surface 
at  places  where  muscles  were  attached.  It  is  cylindrical  in  shape 
above,  but  triangular  below.     At  its  lower  extremity  the  bone 


Fig.  22. —  Dorsal  view  of  right  scapula. 
(Drawn  by  T.  W.  P.  Lawrence. ) 


40 


Elementary  Physiology, 


becomes  broader  from  side  to  side,  and  has  two  articular 
surfaces  called  the  trochlea  and  capitelhim.  These  articular 
surfaces  are  both  convex  from  before  backward ;  but  the 
trochlea,  which  articulates  with  the  ulna,  is  concave  from  side 
to  side  ;  while  the  capitellum,  which  articulates  with  the  radius, 


Fig. 


23- 


-View  from  before  of  the  articulations  of  the  shoulder-bones. 
Thomson.")     \ 


(Allen 


I,  acromio-clavicular  articulation  ;  2,  conoid,  and  3,  trapezoid  part  of  the  coraco-clavicular 
ligament ;  4,  near  the  suprascapular  ligament ;  5,  on  the  coracoid  process,  points  to 
the  coraco-acromial  ligament ;  6,  capsular  ligament  of  the  shoulder-joint ;  7,  coraco- 
humeral  ligament ;  above  6,  an  aperture  in  the  capsular  ligament  through  which  the 
synovial  membrane  is  prolonged  under  the  tendon  of  the  subscapularis  muscle ; 
8,  tendon  of  the  long  head  of  the  biceps  muscle  ;  9,  right  half  of  the  interclavicular 
ligament ;  10,  interarticular  fibro-cartilage  of  the  sterno-clavicular  articulation ; 
II,  costo-clavicular  ligament ;  12  and  13,  cartilage  and  small  part  of  the  second  and 
third  ribs  attached  by  their  anterior  chondro-sternal  ligaments. 


is  convex  in  this  direction  also.  The  upper  ends  of  the  ulna 
and  radius  bear  articular  surfaces  to  match  these  surfaces  on 
the  humerus  (see  Fig.  25).  The  surface  on  the  ulna,  which 
articulates  with  the  humerus,  is  convex  from  side  to  side,  to 
suit  the  concavity  in    this  direction   of  the   trochlea   of  the 


The  Skeleton  and  its  Articulations. 


41 


humerus,  and  correspondingly  concave  from  above  downwards. 
The  surface  by  which  the  head  of  the  radius  articulates  with 


Fig.  24. —  RiffVit  humerus,  from  before. 
CDrawn  by  T.  W.  P.  Lawrence.) 


Fig.  25. — Anterior  view  of  the  right  radius 
and  ulna  in  supination  of  the  hand. 
(Drawn  by  T.  W,  P.  Lawrence.) 


42 


Eleinentaiy  Physiology. 


the  himierus  is  circular  and  slightly  hollowed  out,  so  as  to  suit 

the  convexity  of  the  capitellum. 

Of  the  two  bones  of  the  forearm  (see  Fig.  25),  the  ulna  is 

large  at  the  elbow  and  tapers  towards   the  wrist;    while  the 

radius  is  smaller  at  the  elbow  and  becomes  more  massive  at 

the  v/rist. 

The  radius   and   ulna   articulate  with  each  other   both  at 

elbow  and  wrist.  The  articulations  are  such  as  to  admit  of  a 
rotation  of  the  radius  to  a  certain  extent 
around  the  ulna.  The  head  of  the  radius 
at  the  elbow  is  held  in  position  by  the 
orbicular  ligament  (see  Fig.  25).  Around 
the  head  is  a  smooth  articular  surface  which 
moves  during  rotation  against  a  concave 
facet  on  the  ulna.  At  the  wrist  there  is 
a  convex  articular  facet  on  the  ulna,  and 
a  concave  facet  on  the  radius.  The 
movement  of  the  radius  round  the  ulna 
brings  about  pronation  and  supination  of 
the  hand.  In  supination  the  bones  are 
uncrossed;  in  this  position,  with  the  fore- 
arm held  horizontal,  the  palm  of  the  hand 
is  uppermost.  In  pronation  the  lower 
end  of  the  radius  is  crossed  inwards  over 
^    .    ,       .      the   ulna,  and  the   back    of   the   hand   is 

Fig.  26. — Sagittal  section 

of   the   elbow  joint  uppcrmost.     lu  the  movcment  from  supi- 

through  the  great  sig-  .  .  .  , 

moid   cavity  of  the  nation  to  pronation,  rotation  takes  place  at 

ulna  and  the  trochlear      .  ...         ,  ,•       i    j-  i      i 

.surface  of  the  hume-  thc  radio-umar  articuiations,  and  these  are 

son.)   \    ^^       °"^"   hence  termed  pivot  joints.    The  movement 

I,  cu.t  surface  of  the  hume-   between  the  humcius   and  ulna  is  purely 

rus  ;     2,    that   of    the  .  in*  i      i  •      • 

ulna;  3,  posterior  part,     OnC    01     CXtenSlOll    and    lleXlOn,    and    this    IS 

the  synovLTcaSfty  of  brought  about  by  the  hinge  joint  formed 
?gaL1nV;  V'tendon  by  thcsc  two  boncs.  The  elbow  joint  is 
°^i?at^the^ioweTenci  hcncc  a  compouud  joint,  including  both  a 
of  the  oblique  liga-  hinge  joint  (humcrus  and  ulna)  and  a  pivot 

joint  (radius  and  ulna). 
The  movements  at  the  wrist  are  allowed  to  take  place  by 
a  very  complicated  series  of  articulations  (see  Fig.  27),  partially 


The  Skeleton  and  its  Articulations. 


43 


between  the  radius  and  three  of  the  carpal  bones,  and  partially 
between  the  carpal  bones  themselves.  The  ulna  takes  no 
part  in  the  wrist  joint,  being 
shut  off,  as  shown  in  the  figure 
(Fig.  27),  by  a  strong  liga- 
mentous band.  The  articular 
surface  of  the  radius  which 
touches  the  carpal  bones  is 
concave  both  from  before  back- 
ward and  from  side  to  side, 
and  those  carpal  bones  which 
articulate  with  it  together  fur- 
nish a  correspondingly  convex 
surface. 

The  carpal  bones,  as  shown 
in  the  figure  (Fig.  27),  are  ar- 
ranged in  two  rows ;  synovial   jj  ■  l^ 
cavities    are    present    between     ^'^'^ 

them,  and  there  is  a  consider-   f^^_  37.-Section  of  the  inferior  radio- 
able  amount  of   didinsr    motion  "^^^^'   radio-carpal,    intercarpal     and 
^            ^  carpo-metacarpal  articulations.   (Allen 

possible  between   the  first  and        Thomson.)  ^ 

QprnnH    rowc    hnt    nnlv   n     Qlicrhl-     i.-  triangular   fibro-cartilage ;  2,  placed  on 
SeCOna    rO^^S,    out    Oni}    a    Sngnt  the  ulna,  points  to  the  synovial  cavity 

amount  between  bones  of  the 
same  row. 

Distally^  the  carpal  bones 
01  the  second  row  articulate 
with  the  metacarpal  bones, 
which  are  the  bones  underlying 
the  palm  of  the  hand.  The 
amount  of  movement  between 
the  metacarpal  bones  of  the 
four  fingers  and  the  carpal 
bones  is  not  large,  although  a 
small  amount  of  movement 
inward  of  the  fourth  and  fifth  - 


of  the  inferior  radio-ulnar  articulation  ; 
3j  external  lateral,  and  4,  internal 
lateral  ligament,  and  between  them 
the  sjmovial  cavity  of  the  wTist ;  5, 
scaphoid  bone ;  6,  lunar ;  7,  pyra- 
midal ;  8,  8,  upper  portion,  and  8',  8', 
lower  portion  of  the  general  sj-novial 
cavity  of  the  intercarpal  and  carpo- 
metacarpal articulations  ;  between  5 
and  6,  and  6  and  7,  the  interosseous 
ligaments  are  seen  separating  the 
carpal  articular  cavity  from  the  wrist- 
joint  ;  between  the  four  carpal  bones 
of  the  lower  row,  and  between  the 
magnum  and  scaphoid,  the  inter- 
osseous ligaments  are  also  shown  ;  the 
upper  division  of  the  s>-novial  cavity 
communicates  with  the  lower  between 
10  and  II,  and  between  11  and  12  ;  x, 
marks  one  of  the  three  interosseous 
metacarpal  ligaments  ;  9',  separate 
synovial  cavity  of  the  first  carpo- 
metacarpal articulation  ;  13,  first,  and 
14,  fifth  metacarpal  bone. 


'  The  words  "  proximal  "  and  "' distal"  are  used  to  denote  parts  re- 
spectively nearer  and  farther  from  the  central  part  of  the  body. 

-  The  metacarpal  and  metatarsal  bones  are  numbered  from  the  thumb 


44 


Elementary  Physiology. 


metacarpals  takes  place  in  closing  the  hand.     The  first  meta- 
carpal bone  (that  of  the  thumb)  possesses  a  separate  synovial 

cavity  (see  9',  Fig.  27)  between 
its  proximal  end  and  the  bone 
of  the  carpus  with  which  it 
articulates  (the  trapezium).  This 
joint  is  what  is  known  as  a 
saddle  joints  the  two  surfaces 
concerned  being  respectively 
convex  and  concave  in  directions 
at  right  angles  to  one  another, 
and  fitting  against  each  other 
saddle  fashion.^  The  four  meta- 
carpal bones  of  the  fingers  are 
united  at  their  distal  extremities 
by  a  strong  band  of  ligament 
{tf'ansverse  metacai-pal  ligament) 
which  prevents  any  great  move- 
ment of  these  bones  with  respect 
to  one  another,  and  strengthens 
the  hand.  This  ligament  is 
shown  in  Fig.  28,  which  also 
shows  a  front  view  of  the  liga- 
ments of  the  wrist  joint. 

The  joints  between  the  meta- 
carpal bones  of  the  four  fingers 
^'  'TlJTltt^oTi:!';:^.^^.    and  the  first  row  of  phalanges 
the  lower  end  of  the  radius,  the    Imetacarpo-phalaii^eal  joints)  are 

anterior    radio-carpal    ligament ;    3.  ^                   -^     -^  o          ^             / 

scaphoid  bone  ;  '  4,  pisiform  ;   5,  tra-  tcmied  C07ldyloid  j ointS.     In  thcSC 

pezium ;  6,  unciform ;  7,  os  magnum,  _     .  .                       ^           r       1 

with  most  of  the   deeper  ligaments  jointS     the  dlStal      Cnds     of     the 
uniting  these  bones;    I,  first  meta- 

carpo-phalangeal    articulation    with  metacarpal  boneS      arC      COllVCX 

its  external  lateral  ligament;  II  to  ,      ,1         ^  ^      r             1        i             j 

V,  transverse  metacarpal  ligament  ;  both      frOm  betorC      backwardS, 

luiSL^rXU^rgers^  tCtt^eS    and  from  side  to  side  ;  while  the 
l&Te?naT:r5ytvisiSe'^  '"'"'    proximal   cnds  of  the  first  row 

and  great  toe  respectively,  so  are  the  phalanges  (finger  and  toe  bones)  of 
the  hand  and  foot. 

^  A  saddle  joint  is  so  called  because  of  its  resemblance  to  that  made  by 
a  horseman  with  his  saddle  ;  it  may  be  represented  by  holding  the  thumb 


Fig.  28. — General  view  of  the  articula- 
tions of  the  wrist  and  hand  from 
before.     \ 


The  Skeleton  and  its  Articulations.  45 

of  phalanges  are  correspondingly  concave.  There  is  thus 
allowed  a  certain  amount  of  movement  in  any  direction,  so 
that  each  finger  is  capable  of  a  certain  amount  of  angular 
rotation  (verify  this),  as  if  the  joint  were  a  ball  and  socket.^ 
The  interphalangeal  joints  in  the  fingers  are  simple  hinge 
joints  which  allow^  only  bending  (flexion)  and  straightening 
(extension). 

In  the  thumb  the  metacarpo-phalangeal  joint  is  a  hinge, 
and  not  a  condyloid,  joint,  the  function  of  the  latter  being 
taken  on  by  the  saddle  joint  described  above,  between  the 
first  metacarpal  and  trapezium  bones.  The  joint  between  the 
phalanges  of  the  thumb  is  a  hinge  joint. 

The  Lower  Limb. 

The  fe??mr,  or  thigh-bone,  is  the  longest  bone  in  the 
skeleton,  and  extends  from  the  hip  to  the  knee.  Since  in 
the  normal  standing  position,  with  the  feet  close  together, 
the  knees  are  also  close,  while  the  acetabula  into  which 
the  heads  of  the  femora  are  inserted  are  at  opposite  sides 
of  the  pelvis,  it  follows  that  each  femur  in  this  position 
inclines  somewhat  inward  as  it  descends.  At  the  same  time  it 
also  inclines  slightly  backward.  The  amount  of  this  inclination 
inwards  is  greater  in  w^oman  than  in  man.  The  femur  (see 
Fig.  31),  for  descriptive  purposes,  is  divided  into  superior  ex- 
tremity, shaft,  and  inferior  extremity.  The  superior  extremity 
iijcludes  the  head,  neck,  and  trochanters.  The  head  is  spherical 
in  shape,  forming  more  than  half  a  sphere,  is  covered  with 
articular  cartilage,  and  forms  the  ball  of  the  "  ball  and  socket " 
of  the  hip  joint.  The  neck  is  a  stout  rounded  mass  of  bone 
connecting  the  head  with  the  shaft  and  meeting  the  shaft  at  an 
angle  of  125°.     The  great  2in6.  small  trochanters  (see  Fig.  31) 

as  far  as  possible  from  the  fingers  of  the  hand  and  placing  the  two  forks  so 
formed  in  contact  at  right  angles. 

^  There  is  this  difference,  however,  between  a  ball-and-socket  and  a 
condyloid  joint,  that  in  the  former  one  of  the  bones,  viz.  that  bearing  the 
ball  surface,  can  rotate  in  the  socket  formed  by  the  other  ;  while  in  the 
condyloid  joint  there  is  no  such  rotation,  the  angular  rotation  being  due  to 
a  free  gliding  motion. 


46  '   Elementary  Physiology. 

are  two   roughened  prominences  or  ridges   of  bone  situated 


INT.  FIBRO-CART. 


SEMILUNAR 

PATELLAR 

FACET 


EXT.  TIBIAL        I   — 
SURFACE 


INTERNAL 
TUBEROSITY 


INT.  TIBIAL 
SURFACE 


INTERCONDYLAR     FOSSA 
POST.    CRUC.    LIST. 

Fig.  29. — Lower  extremity  of  right  femur,  from  below. 
(Drawn  by  T.  W.  P.  Lawrence.)    | 

where  the  neck  joins  the  shaft,  which  serve  for  the  attachment 
of  muscles  to  the  bone. 

The  shaft  of  the  femur  is  not  quite  straight,  but  shghtly 


ANT.   CRUC.   LIGT. 
INT.   FIBRO-CART. 


TUBERCLE 

EX.T.  FIBRO-CART. 


ILIO-TIBIAL 

BAND 


INT.    FIBRO-CART.  EXT.    FIBRO-CART. 

POST.    CRUC.    LIGT. 

Fig.  30. — Upper  e.xtremity  of  the  right  tibia,  from  above. 
(Drawn  by  T.  W.  P.  Lawrence.)    f 

curved  from  above  downwards,  with  the  convexity  forwards.    It 
is  nearly  cylindrical  in  section  in  the  middle  of  its  length,  but 


The  Skeleton  and  its  Articulations .  47 

becomes  very  much  broadened  laterally  at  its  lower  end,  where 
it  passes  into  the  inferior  extremity. 


FOSSk 

INTCBARTIC.- 
LICT 


POST.  CnuCL'i    UuT.  ANT.  cnuCtAU  UCT. 

lUTCRCOKOVLAR      NOTCH 

Fjg.  31. — Right  femur,  from  behind. 
(Drawn  by  T.  W.  P.  Lawrence.) 


Fig.  32^Right  tibia  and  fibula,  from  behind 
(Drawn  by  T.  W.  P.  Lawrence.) 


48  Elementary  Physiology. 

The  mferior  extremity  (see  Fig.  29)  is  the  broadened-out 
lower  end  of  the  shaft;  it  forms  two  eminences,  called  the 
external  and  internal  condyles^  which  are  united  in  front,  but 
separated  behind  by  a  notch,  or  fossa  {intercondylar  fosse).  The 
surfaces  of  the  condyles  are  convex  both  from  before  backward 
and  from  side  to  side,  while  between  them  in  front  is  a  concave 
hollow,  which  extends  to  some  extent  up  the  front  of  the  lower 
extremity  of  the  bone.  All  this  surface  is  covered  by  smooth 
articular  cartilage.  The  grooved  surface  between  the  condyles 
articulates  with  the  patella  (or  knee-pan),  the  convex  eminences 
of  the  condyles  with  the  nearly  flat  upper  articular  surface  of 
the  tibia  (shin-bone),  and  the  semiltmar  jibro-cartilages  which 
are  the  interarticular  cartilages  of  the  knee  joint. 

The  articulation  of  the  head  of  the  femur  with  the  hip-bone 
has  already  been  described  (see  p.  37).  The  lower  extremity 
forms  part  of  the  knee  joint,  which  is  shown,  partially  in  section, 
in  the  accompanying  figures. 

The  bones  taking  a  share  in  the  formation  of  the  knee  joint 
are  the  femur,  tibia,  and  patella.  The  patella  is  what  is  known 
as  a  sesamoid  bone^ — that  is,  a  bone  formed  in  the  tendon  of  a 
muscle,  usually  where  it  passes  over  or  bends  round  a  bony 
surface.  The  patella  is  shaped  like  a  triangle  with  rounded  off 
angles,  and  is  placed  base  upwards  just  in  front  of  the  knee 
joint  in  the  combined  tendons  of  the  extensor  muscles  which 
straighten  (or  extend)  the  leg  at  the  knee.  It  serves  the  double 
purpose  of  protecting  the  knee  joint  in  front  and  of  hardening 
and  strengthening  the  extensor  tendon  where  this  bends  round 
the  knee. 

Between  the  opposed  articular  surfaces  of  the  femur  and 
tibia,  which  are  the  two  chief  bones  in  the  knee  joint,  are  inter- 
posed the  semihinar  fiJ)ro-cartilages..  These  are  two  crescentic- 
shaped  plates  with  a  free  surface  both  above  and  below,  and 
are  thick  at  their  outer  border  (which  is  attached  in  each  case 
to  the  capsule  of  the  joint),  but  thin  away  to  an  edge  at  their 
inner  margin  so  as  to  leave  part  of  the  upper  articular  surface 
of  the  tibia  free  in  the  centre.  This  portion  of  the  articular 
surface  of  the  tibia  comes  in  contact  with  that  of  the  femur. 
The  two  ends  of  each  semilunar  cartilage  are  attached  to  the 


TJie  Skeleton  and  its  Articulations. 


49 


non-articular  part  of  the  upper  extremity  of  the  tibia  which  is 
interposed  between    the  two  articular  surfaces  (see  Fig.  30). 


Fig.  33. — The  superficial  parts  of  the  knee  joint  removed,  and  the  external  condyle  of 
the  femur  sawn  off  obliquely-,  together  with  half  the  patella,  so  as  to  expose  both 
the  crucial  ligaments  together,     (Allen  Thomson.)    ^ 

In  A,  the  parts  are  in  the  position  of  extension,  in  B,  that  of  flexion,  the  figures  being 
designed  to  show  the  different  states  of  tension  of  the  crucial  ligaments  in  these 
positions,  i,  sa^ra  surface  of  femur;  2,  sawn  surface  of  patella;  3,  ligamentum 
patell2e;_4,  anterior  or  external  crucial  ligament,  tense  in  A,  and  relaxed  in  B; 
5,  posterior  or  internal  crucial  ligament,  partly  relaxed  in  A,  tense  in  B  ;  6,  internal^ 
and  7,  external  semilunar  fibro-cartilage  ;  3,  transverse  ligament ;  9,  articular  surface 
of  the  tibia,  extending  behind  the  external  semilunar  fibro-cartilage  ;  10,  on  the 
head  of  thefibula,  points  to  the  anterior  superior  tibio-peroneal  ligament;  11,  upper 
part  of  the  interosseous  membrane. 

The  semilunar  cartilages'  are*  thus  crescent-shaped  wedges 
which  adapt  the  articular  surfaces  of  tibia  and  femur  to  each 
other.  In  the  accompanying  figures  of  the  knee  joint  there  are 
also  shown  the  two  crucial  ligaments  which  connect  the  femur 
and  tibia,  and,  besides  strengthening  the  union,  prevent  over- 
extension. 

The  movement  at  the  knee  is  not  purely  that  of  a  hinge 
joint,  although  the  effect  is  much  the  same  ;  but  is  a  compound 
of  gliding  and  rolling  of  the  condvles  of  the  femur  on  the  tibia 


50  Elementary  Physiology. 

and  semilunar  cartilages,  accompanied  (at  the  end  of  extension; 
or  beginning  of  flexion)  by  a  small  amount  of  rotation. 

The  bones  of  the  lower  leg  are  the  tibia  and  fibula  (see 
Fig.  32) ;  of  these  the  tibia  is  the  stronger  and  more  massive. 
It  transmits  all  the  weight  of  the  body  above  it  to  the  ankle, 
for  the  fibula  takes  no  part  in  the  knee  joint,  and  the  tibia 
furnishes  the  greater  part  of  the  surface  at  ^;he  ankle  joint  for 
articulation  with  the  astragalus^  that  bone  of  the  foot  which 
articulates  at  the  ankle. 

The  tibia  and  fibula  are  of  almost  equal  length,  the  head  of 
the  fibula  lying  a  little  lower  than  that  of  the  tibia  at  the  knee, 
while  the  fibula  lies  a  little  lower  at  the  ankle.  It  here  forms 
the  external  malleolus^  a  prominence  of  bone  at  the  outer  side 
of  the  ankle.  The  corresponding  prominence  on  the  inner 
side  is  furnished  by  the  tibia,  and  is  known  as  the  internal 
malleoliLs. 

In  position,  the  tibia  lies  to  the  inside  and  somewhat 
anteriorly  to  the  fibula.  The  two  bones  articulate  both  at  their 
upper  and  lower  extremities  ;  but  there  is  practically  no  move- 
ment at  either  articulation,  so  that  there  is  nothing  in  the  leg, 
corresponding  to  pronation  and  supination  of  the  forearm. 
The  tibia  and  fibula  are  united  by  strong  interosseous  ligaments. 
The  fibula  is  a  long  slender  bone  which  serves  to  strengthen 
the  ankle  joint.  It  furnishes  to  this  joint  an  articular  surface 
situated  on  the  inner  surface  of  the  external  malleolus.  The 
fibula  gives  attachment  to  some  of  the  leg  muscles.  Others  of 
these  muscles  are  attached  to  the  tibia  and  to  the  lower 
extremity  of  the  femur  ;  they  cause  the  movements  which  take 
place  at  the  ankle  joint  and  those  of  the  foot  and  toes. 

The  ankle  joint  is  formed  by  the  tibia  and  fibula  above,  and 
the  astragalus  beneath.  A  section  across  the  joint  from  side  to 
side  is  shown  in  Fig.  34,  and  another  from  before  backward  is 
seen  in  Fig.  35.  The  movements  are  those  of  flexion^  in 
which  the  foot  is  bent  upwards  in  front  towards  the  leg,  and 
extension^  in  which  the  foot  is  depressed  and  brought  into  a  line 
with  the  leg.  The  total  range  of  movement  is  about  a  right 
angle. 

There  are  seven  bones  in  the  tarsus  (see  Fig.  36),  named 


TJie  Skeleton  and  its  Articulations. 


51 


respectively  :  i,  astragalus ^  which  forms  the  lower  articulation 
of  the  ankle  joint ;  2,  calcanetnn^  or  os  calcis^  which  lies  beneath 
the  astragalus  and  projects  backward  to 
form  the  heel,  to  which  the  tcndo  A  chillis  is 
attached  conveying  the  pull  of  the  calf 
muscles  to  the  foot ;  3,  navicular^  or 
scaphoid^  which  lies  in  front  of  the  astraga- 
lus at  the  inner  side  of  the  foot  and 
articulates  with  this  bone  and  with  the 
three  cuneiform  bones  which  lie  in  front  of 
it;  4,  cuboid,  which  is  situated  on  the 
outside  of  the  foot  in  front  of  the  calcaneum, 
with  which,  and  the  fourth  and  fifth  meta- 
tarsal bones,  it  articulates;  5,  6,  7,  the 
internal,  middle,  and  external  cimeiforms ^ 
which  lie  in  front  of  the  navicular  on  the 
inner  side  of  the  foot  between  that  bone 
and  the  first,  second,  and  third  metatarsal 
bones  respectively. 

The  bones  of  the  tarsus,  together  with 
the  five  metatarsal  bones,  form  an  arch 
both  from  before  backward,  and  from  side 
to  side,  which  is  known  as  the  arch  of  the 
foot. 

There  is  very  little  movement  between 
the  tarsal  and  metatarsal  bones,  save  a 
small  amount  of  gliding  motion.  A  small 
amount  of  rotation  can  take  place  between 


Fig 


astragalus  and  calcaneum. 


34. — Section  of  the 
right  ankle  joint  near 
its  middle,  and  of 
the  posterior  astra- 
galo-calcaneal  articu- 
lation, viewed  from 
before.  (Allen Thom- 
son.)    \ 

internal ;  2,  external 
malleolus ;  3,  placed 
on  the  astragalus  at 
the  angle  between  its 

•  superior  and  its  ex- 
ternal surfaces  ;  4,  in- 
lerior  interosseous 
tibio-fibular  ligament ; 
5,  internal  lateral 
ligament  of  the  ankle 
joint ;  6,  sustentacu- 
lum tali  ;  7,  calcaneo- 
iibular  or  middle  part 
of  the  external  lateral 
ligament ;  8,  inner 
part  of  the  inter- 
osseous astragalo-cal- 
caneal  ligament ;  9, 
tuberosity  of  the  cal- 
caneum. 


movement    takes 


The  chief  movement  is  termed  inversion 
and  eversion.  In  inversion  the  outer  side 
of  the  foot  is  depressed  and  the  sole 
turned  inward ;  in  eversion  the  opposite 
place. 

The  movements  between  metatarsal  bones  and  phalanges 
and  the  interphalangeal  movements  are  similar  to  those  of  the 
hand ;  but  the  range  of  movement  is  somewhat  less,  especiall}- 
in  the  case  of  the  hallux,  or  great  toe. 


52 


Elementary  Physiology. 


The  bones  of  the  foot  are  shown  in  the 
figures  (A  and  B,  Fig.  36). 


accompanying 


Fig.  35. — Sagittal  section  of  the  ankle-joint  and  articulations  of  the  right  foot,  a  little 
to  the  inside  of  the  middle  of  the  great  toe.     (Allen  Thomson.)    \ 

1,  synovial  cavity  of  the  ankle  joint ;  2,  posterior  astragalo-calcaneal  articulation ; 
3,  3',  astragalo-calcaneo-navicular  articulation  :  the  interosseous  ligament  is  seen 
separating  2  from  3' ;  4,  inferior  calcaneo-navicular  ligament ;  5,  part  of  the  long 
plantar  ligament ;  6,  naviculo-cuneiform  articulation ;  7,  first  cuneo-metatarsal 
articulation  ;  8,  first  metatarso-phalangeal  articulation  ;  .  9,  section  of  the  inner 
sesamoid  bone;  10,  interphalangeal  articulation;  11,  placed  on  the  calcaneum, 
indicates  the  bursa  between  the  upper  part  of  the  tuberosity  of  that  bone  and  the 
tendo  Achillis. 

This  brief  outHne  of  the  structure  of  the  skeleton  may  be 
concluded  with  the  following  classification  of  the  articulations  : — 

Articulations  are  divided  into  two  classes — viz.  synarthroses, 
or  continuo2Ls  articulations ;  and  diarthroses^  or  discontinuous 
articulations. 

In  a  synarthrosis  the  bony  surfaces  are  fixed  together  either 
directly  or  by  some  interposed  substance,  such  as  a  disc  of 
cartilage.  The  two  most  frequent  forms  of  synarthrosis  found 
in  the  body  are  the  suture  and  the  symphysis. 

The  suture  is  met  with  only  in  the  case  of  the  skull-bones. 
In  this  form  of  articulation  the  opposed  surfaces  of  bone 
practically  come  in  contact,  being  only  separated  by  a  thin 
layer  of  fibrous  tissue.  The  edges  of  the  bones  where  they 
come  in  contact  are  often  deeply  indented  or  serrated^  so  as  to 
firmly  interlock  with  each  other.  In  other  cases  the  edges  are 
thin  and  bevelled,  to  meet  each  other,  forming  a  squamotis 
suture  ;  or  they  may  be  grooved,  forming  2i  grooved  suture. 


The  Skeleton  and  its  Articulations. 


30 


In  a  symphysis  the  bones  are  united  by  a  disc  of  fibro- 
cartilage  which  acts  as  an  elastic  pad,  as  has  been  described  in 
the  case  of  the  bodies  of  the  vertebra  and  of  the  two  pubic 
bones. 

Synchondrosis  is  a  term  applied  to  growing  bones  separated 
by  cartilage  which  aften^-ards  become  united  :  as  in  the  case  of 


U3 


Fig.  36.— The  bones  of  the  right  foot :  A,  from  above  ;  B,  fiom  below. 
(Alien  Thomson.)    i 

a,  navicular  bone  ;  b,  astragalus  ;  c,  os  calcis  ;  d,  its  tuberosity  ;  ^,  internal  cuneiform  ; 
f,  middle  cuneiform ;  g,  external  cuneiform  ;  h,  cuboid  bone  ;  I  to  V,  the  metatarsal 
bone  ;  i,  3,  first  and  last  phalanges  of  the  great  toe  ;  i,  2,  3,  first,  second,  and  third 
phalanges  of  the  second  toe- 


the  three  bones  which  later  in  life  unite  to  form  the  hip-bone ; 
and  as  in  the  case  of  the  occipital  bone  at  the  base  of  the  skull 
which  early  in  life  consists  of  several  distinct  bones.  As  the 
component  bones  grow  they  approach  one  another,  and  finally 


54  Elementaiy  Physiology. 

their  edges  unite  and  one  bone  is  formed,  the  process  of  union 
being  described  as  synostosis. 

Syndesmosis  is  a  term  applied  when  the  bones  are  united 
by  an  interosseous  Hgament,  as  is  the  case  in  the  lower 
articulation  between  tibia  and  fibula. 

In  diarthrosis,  or  discontinuous  articulation,  the  opposed 
surfaces  of  bone  remain  distinct,  they  are  surrounded  by 
synovial  cavities,  and  there  is  more  or  less  freedom  of 
movement  between  them.  These  are  the  true  joints  of  the 
body,  and  are  divided  into  the  following  classes  : — 

1 .  Gliding  joints  in  which  the  surfaces  are  nearly  flat  and 
capable  of  gliding  to  a  limited  extent  over  each  other ;  examples 
are  found  in  the  carpal  and  tarsal  bones. 

2.  Hinge  joints  in  which  the  articular  surfaces  are  arranged 
to  permit  a  hinge-like  movement  by  which  the  joint  can  be 
straightened  (extension)  or  bent  (flexion).  One  surface  is 
usually  pulley-shaped  and  the  other  correspondingly  ridged. 
Examples  are  the  articulation  between  humerus  and  ulna  at  the 
elbow  joint  and  the  ankle  joint. 

3.  Condyloid  joints  in  which  rounded  surfaces  are  present, 
one  being  convex  and  the  other  concave.  Angular  movement 
in  any  direction  is  allowed  by  such  a  joint,  and  hence  cir- 
cumduction or  angular  rotation.  The  movements  of  the 
fingers  at  the  joint  between  the  metatarsal  bone  and  first 
phalanx  illustrate  the  action  of  such  a  joint.  Similar  joints 
exist  at  the  corresponding  position  between  the  metatarsal 
bones  and  the  first  row  of  phalanges  in  the  foot. 

4.  Saddle  joints  in  which  the  opposed  surfaces  are  saddle- 
shaped  {i.e.  convex  in  one  direction  and  concave  in  another 
at  right  angles  to  it)  and  placed  so  that  the  convexity  of  one  is 
applied  to  the  concavity  of  the  other.  Such  an  articulation  is 
found  between  the  trapezium  and  the  metacarpal  bone  of  the 
thumb. 

5.  Pivot  joints  in  which  one  articular  surface  is  either 
cylindrical  or  cone  shaped,  and  the  other  is  correspondingly 
concave,  as  in  the  case  of  the  articulation  between  radius  and 
ulna  at  the  elbow,  and  that  between  atlas  and  axis  by  means  of 
which  the  head  is  rotated  (see  pp.  22,  42). 


The  Skeleton  and  its  Articulations.  55 

6.  Ball-and-socket  joints  in  which  one  articular  surface  is 
spherical  or  nearly  so,  and  the  other  is  hollowed  out  to  form 
a  cup  to  receive  it.  Examples  have  been  described  in  the 
shoulder  and  hip  joints  (see  pp.  37,  39). 

In  some  cases  a  joint  cannot  accurately  be  described  as 
belonging  rigorously  to  any  one  of  these  six  classes,  as  in  the 
case  of  the  articulation  of  the  lower  jaw  with  the  temporal 
bone,  where,  as  already  described,  there  is  a  hinge  movement  of 
the  condyle  of  the  jaw  on  the  lower  surface  of  the  interarticular 
cartilage,  and  at  the  same  time  a  gliding  movement  of  the 
upper  surface  of  the  interarticular  cartilage  forward  out  of  the 
groove  on  the  temporal  bone  on  to  the  eminence  in  front  of  it. 
Again,  at  the  knee  joint,  while  the  general  movement  is  a  hinge 
one,  the  motion  is  accomplished  by  a  mixed  motion  of  gliding, 
rolling,  and  rotating. 


CHAPTER   III. 

THE  MUSCULAR  SYSTEM. 

The  jointed  bony  system,  or  skeleton,  of  which  the  structure 
has  been  sketched  above,  is  clothed  or  covered,  except  at 
certain  parts  where  the  bones  are  subcutaneous,  by  masses  of 
flesh,  which  are  made  up  of  muscles,  sheathed  in  coverings  of 
connective  tissue  known  2.^  fascia. 

The  muscles  are  masses  of  soft  tissue  which  are  capable 
of  changing  their  form  under  the  influence  of  impulses  con- 
veyed to  them  along  the  nerves  from  the  nervous  system.  We 
shall  presently  see  that  the  muscles  are  built  up  of  elongated 
microscopic  elements  called  muscle  fibres.  When  the  nerve 
impulse  reaches  the  muscle  each  fibre  cojitracts.,  or  becomes 
shortened  in  length,  and  at  the  same  time  thickened  so  as 
to  preserve  an  unaltered  volume.  The  result  of  this  is  that 
the  muscle  as  a  whole  shortens  or  contracts  in  the  direction 
of  its  constituent  fibres,  and  swells  up  in  a  direction  at  right 
angles  to  this.  The  effect  of  such  a  muscular  contraction  will 
depend  upon  the  attachments  of  the  muscle. 

One  of  the  most  usual  forms  of  attachment  is  to  two  bones, 
one  on  each  side  of  a  joint.  In  such  a  case  the  muscle  is 
often  elongated  and  spindle-shaped,  with  a  thick  middle  part 
forming  the  belly,  and  thins  out  at  each  end  into  a  fibrous  cord 
or  band,  called  the  tendon  of  the  muscle.^  The  tendon  is 
strongly  attached  to  the  bone,  and  when  the  muscle  contracts 
exerts  a  strong  pull  upon  it.  An  example  of  such  a  muscle 
is  the  biceps  ^  of  the  upper  arm,  which  on  contracting  bends  or 

^  A  tendon  must  not  be  confused  with  a  ligament  ;  both  are  fibrous  bands, 
but  the  tendon  is  always  connected  to  a  muscle  at  one  end,  while  the  liga- 
ment is  not. 

^  So  called  because  it  divides  above  into  two  parts. 


Elerfientary  Physiology.  57 

flexes  the  elbow  joint.  The  thickening  of  this  muscle  as  it 
contracts  can  be  felt  as  the  arm  is  "  put  up "  or  bent  at  the 
elbow. 

In  other  cases  the  muscles  are  arranged  in  broad  sheets,  which  some- 
times encircle  a  cavity.  Examples  of  such  muscles  are  the  sheets  of  muscle 
which  form  the  abdominal  wall,  and  the  diaphragm  or  midriff  which 
separates  the  thoracic  from  the  abdominal  cavity.  When  these  sheets  of 
muscle  contract  they  compress  the  cavity  towards  which  their  concave 
surface  is  turned.  If  the  diaphragm  contracts,  for  example,  it  compresses 
the  contents  of  the  abdominal  cavity,  unless  the  pressure  is  relieved  by  the 
relaxation  of  the  muscles  of  the  abdominal  wall  in  front  ;  at  the  same  time, 
a  suction  is  exercised  on  the  contents  of  the  thoracic  cavity,  which  is  re- 
lieved by  the  air  entering  and  filling  the  lungs.  This -is  the  usual  action  of 
the  diaphragm  in  breathing,  but  if  both  diaphragm  and  abdominal  muscles 
contract  at  the  same  time,  the  contents  of  the  abdominal  cavity  are  strongly 
compressed.  This  simultaneous  contraction  takes  place  in  the  process  of 
defcecation,  in  which,  a  ring  of  muscle  at  the  lower  end  of  the  intestine  being 
relaxed,  any  unused  detritus  of  the  food  which  has  not  been  digested  and 
taken  into  the  body  is  ejected  at  the  amis  (the  lower  opening  of  the 
alimentary  canal)  by  means  of  the  pressure  so  exerted. 

The  muscles  connected  with  the  skeleton  {skeletal  viuscles) 
hence  vary  greatly  in  form  according  to  the  work  which  they 
are  called  upon  to  perform.  A  complete  description  of  the 
skeletal  muscles  and  their  actions  w^ould  fill  a  greater  space 
than  this  entire  volume,  but  some  idea  of  their  general  arrange- 
ment may  be  gathered  from  the  following  diagrams  and  their 
accompanying  descriptions.^ 

All  the  muscles  of  the  body  are  not,  however,  connected 
with  the  skeleton.  Some  form  coats  lining  various  tubes  of 
the  body,  and  others  are  found  in  various  organs.  Thus,  the 
heart  is  a  hollow  organ  containing  four  chambers,  the  walls 
of  which  are  composed  of  muscular  tissue.  By  its  contractions, 
this  muscular  tissue  alters  the  volumes  of  these  chambers,  and 
the  alteration  in  volume,  aided  by  an  arrangement  of  valves 
to  be  subsequently  described,  has  the  effect  of  driving  the 
blood  through  the  heart  and  the  system  of  tubes  connected 
with  it.  The  walls  of  these  tubes  (the  arteries  and  veins)  are 
also  hned  with  a  muscular  coat  which  alters  the  bore  or  calibre 
of  the  vessels.     This  muscular  coat  is  strongly  developed  in 

^  See  also  Fig.  15,  p.  29, 


Fig.  37. — Superficial  view  of  the  muscles  of  the  trunk,  from  before.     (Allen  Thomson.) 

I,  sterno-mastoid  of  the  left  side ;  i',  1",  platysma  myoides  of  the  right  side  ;  2,  sterno- 
hyoid ;  3,  anterior,  3',  posterior  belly  of  the  omo-hyoid  ;  4,  levator  anguliscapulas  ; 
4',  4",  scalene  muscles  ;  5,  trapezius ;  6,  deltoid  ;  7,  upper  part  of  triceps  in  the  left 
arm;  8,  teres  minor;  9,  teres  major  ;  10,  latissimus  dorsi  ;  11,  pectoralis  major;  11', 
on  the  right  side,  its  clavicular  portion  ;  12,  part  of  pectoralis  minor ;  13,  serratus 
magnus  ;  14,  external  oblique  muscle  of  the  abdomen  ;  15,  placed  on  the  ensiform 
process  at  the  upper  end  of  the  linea  alba  ;  15',  umbilicus  ;  16,  is  placed  over  the 
symphysis  pubis,  at  the  lower  end  of  the  linea  alba;  above  16,  the  pyramidal  muscles 
are  seen  through  the  abdominal  aponeurosis  ;  14  to  17,  linea  semilunaris  at  the  outer 
border  of  the  rectus  muscle,  the  transverse  tendinous  lines  of  which  are  seen  through 
the  abdominal  aponeurosis;  18,  gluteus  mediiis ;  19,  tensor  vaginae  femoris ;  20, 
rectus  femoris  ;  21,  sartorius  ;  22,  femoral  part  of  the  iUo-psoas  ;  23,  pectineus  ;  24, 
abductor  longus  ;  25,  gracilis. 


'iMI/'f 


rz-- 


is 


rl6 


Fig.  39. — Superficial  muscles  of  the  leg,  seen  from 
the  inner  side.     (After  Bourgery.) 

vastus  internus ;  2,  sartorius  ;  2',  its  tendon,  spreading 
on  the  inner  upper  part  of  the  tibia  ;  3,  gracilis  ;  4,  semi- 
tendinosus  ;  4',  its  insertion  ;  and  between  2'  and  4',  that 
of  the  gracilis  ;  5,  semi-membranosus  ;  6,  inner  head 
of  the  gastrocnemius ;  7,  soleus ;  8,  8',  placed  upon  the 
tendon  Achillis,  point  to  the  tendon  of  the  plantaris 
descending  on  the  inner  side  ;  g,  small  part  of  the_  tendon 
of  the  tibialis  posticus ;  10,  flexor  longus  digitorum ; 
II,  flexor  longus  hallucis ;  12,  tibialis  anticus;  12',  its 
tendon  of  insertion  ;  13,  abductor  hallucis. 


Fig.' 38.— Superficial  muscles  of    the    shoulder   and   upper   limb,  from   before.     (Allen 

Thomson.) 

I,  pectoralis  major,  its  sterno-costal  portion  ;  i',  its  clavicular  portion ;  2,  deltoid,  its 
clavicular  part  ;  2',  its  acromial  part  ;  3,  biceps  brachii ;  3',  its  tendon  of  msertion  ; 
3",  its  aponeurotic  slip ;  4,  brachialis  anticus  ;  4',  its  inner  and  lower  port'on  ;  5> 
long  head  of  the  triceps  ;  5',  inner  head  of  the  same,  seen  arismg  from  behind  the 
intermuscular  septum  ;  6,  pronator  radii  teres  ;  7,  flexor  carpi  radialis  ;  8,  palmans 
longus,  passing  at  8'  into  the  palmar  aponeurosis;  9,  flexor  carpi  ulnaris  ;  10,  10, 
supinator  longus  ;  between  10  and  3',  +,  supinator  brevis  ;  11,  extensorossis  metacarpi 
pollicis;  12,  extensor  brevis  pollicis  ;  13,  lower  part  of  the  flexor  subliniis  digitorum  ; 
14,  flexor  longus  pollicis  ;  15,  small  part  of  the  flexor  profundus  digitorum ;  16, 
palmaris  brevis,  lying  on  the  muscles  of  the  little  finger;  17,  abductor  pollicis. 


6o  Elementary  Physiology. 

the  smaller  arteries,  where  it  is  chiefly  arranged  so  that  its 
fibres  encircle  the  blood-vessel.  When  these  fibres  contract 
the  bore  of  the  vessel  is  diminished,  and  the  blood  cannot 
flow  so  rapidly  through  it;  on  the  other  hand,  when  the 
muscle  fibres  are  relaxed  the  vessel  is  widened,  and  the  blood 
can  flow  through  with  greater  ease. 

To  these  muscle  fibres,  nerve  fibres  pass  {vasomotor  fibres),  and  the 
muscle  fibres  are  kept  in  tone '  by  impulses  sent  to  them  from  the  nervous 
system  along  these  fibres.  In  this  way  the  blood-supply  to  any  part  of  the 
body  is  regulated  by  the  nervous  system.  The  nervous  centres  have  nerve 
fibres  running  both  to  the  centre  from  any  part  {afferent fibres),  and  from 
the  centre  to  the  part  {efferent  fibres) .  When  any  change  takes  place  in  the 
part,  a  nerve  impulse  passes  to  the  centre  by  an  afferent  fibre,  and  then 
a  suitable  reply  is  sent  back  to  the  part  along  an  efferent  fibre.  Suppose, 
for  example,  a  certain  set  of  muscles  are  set  hard  at  work  contracting ; 
then  energy  is  used  up  rapidly,  and  an  increased  supply  of  blood  is  required. 
The  nerve  centre  being  informed  of  this  sends  nerve  impulses  down,  which 
relax  the  muscular  coats  of  the  small  arteries  supplying  these  muscles,  and 
also  stops  impulses  which  had  previously  been  passing  down  and  keeping 
them  constricted,  in  this  way  the  supply  of  blood  to  the  muscles  is  greatly 
increased.  Suppose,  again,  one  commences  eating  something,  saliva  is  at 
once  secreted,  because  impulses  pass  from  nerve  endings  in  the  mouth  up 
to  a  nerve  centre,  and  back  from  there  come  impulses  to  the  cells  of  the 
salivary  glands,  causing  these  to  secrete.  Impulses  are  also  sent  to  the 
arterioles  (small  arteries)  which  supply  the  glands  relaxing  these,  and  giving 
that  bigger  supply  of  blood  to  the  cells  which  is  required  by  them  on 
account  of  their  increased  activity. 

Other  muscle  fibres  form  coats  which  line  the  wall  of  the 
alimentary  canal;  these  cause  the  onward  movement  of  the 
food  undergoing  digestion  by  a  kind  of  contraction  called 
peristalsis.  This  is  a  wave  of  constriction  which  passes  along 
the  tube  and  gradually  shifts  its  contents  onward,  until  what 
is  left  unabsorbed  during  the  passage  is  finally  collected  in 
the  rectttm  (the  last  portion  of  the  alimentary  canal),  from 
which  it  is  ejected  by  the  act  of  defaecation  as  mentioned 
above. 

^  The  tonicity  of  muscles  means  the  degree  of  contraction  at  which  they 
are  kept  by  nervous  action.  A  muscle  is  never  completely  relaxed  ;  it  is 
always  kept  on  the  alert,  as  it  were,  by  the  nerves  supplying  it,  and  when 
a  limb,  say,  is  moved  the  motion  takes  place  by  an  increase  of  this  tonicity 
in  one  set  of  muscles,  and  a  diminution  in  the  other  opposing  set  which 
move  the  limb  in  the  opposite  way. 


The  Muscular  System.  61 

Similar  muscular  coats  are  found  lining  other  vessels  in 
the  body,  such  as  the  urinary  passages.  Muscle  fibres  are 
also  found  in  the  spleen  (see  p.  92),  which  they  cause  to 
contract  at  intervals  of  about  once  a  minute. 

Muscle  is,  then,  that  tissue  in  the  body  which  is  directly 
responsible  for  all  movements,  but  the  nature  of  the  movement 
varies  very  greatly.  In  one  case  it  is  a  movement  of  a  joint 
causing  a  change  in  position  of  the  whole  animal ;  or  of  part 
of  it  relatively  to  the  rest.  In  another  case  it  is  the  mxovement 
of  a  fluid,  such  as  the  circulation  of  the  blood  in  the  blood- 
vessels by  the  contraction  of  the  heart  muscle.  In  another 
case,  the  redistribution  of  the  relative  supply  of  blood  to 
different  parts  by  the  contraction  or  relaxation  of  the  muscle 
fibres  surrounding  the  walls  of  the  arterioles.  In  yet  another 
case,  the  passage  of  food  along  the  alimentary  canal  by  the 
peristaltic  wave  of  contraction  travelling  slowly  along  the  tube. 

All  these  varied  acts  of  muscular  contraction  are  carried 
out  under  the  control  of  the  nervous  system ;  but  while  some 
are  also  under  the  control  of  the  individual,  or,  as  it  is  termed, 
are  vohuitary\  others  are  carried  out  by  the  nervous  system 
acting  automatically,  and  are  said  to  be  involuntary^  since  they 
are  not  under  the  control  of  the  will. 

Muscular  tissue  is  hence  divided  into  two  kinds,  voluntary 
and  involuntary,  and  there  are  other  well-marked  differences 
in  structure  between  the  two  kinds  of  fibre  which  will  presently 
be  described.  Voluntary  muscle  fibres,  for  example,  when 
observed  under  the  microscope,  are  seen  to  be  marked  by 
cross  striations — that  is,  they  show  markings  across  them, 
dividing  the  fibre  into  narroiiw  bands,  alternately  light  and  dark 
(see  Fig.  41).  Such  cross  striation  is  absent  in  involuntary 
muscle  fibre.  Hence  the  terms  cross-striated  or  striped,  and 
plain  or  non-striated  muscle,  are  often  synonymously  used 
instead  of  the  terms  voluntary  and  involuntary  respectively. 
Heart  muscle  is  intermediate  between  these  two  kinds  in  its 
structure,  and  is  accordingly  placed  in  a  class  by  itself  as  cardiac 
muscle.  It  shows  cross  striation,  but  less  perfectly  marked  than 
voluntary  muscle.  Its  contractions  are  not  under  the  control 
of  the  will,  so  that  functionally  it  is  involuntary. 


62 


Elementary  Physiology. 


We  shall,  in  the  first  place,  briefly  describe  the  structure  of 
the  different  kinds  of  muscle,  and  then  the  general  mechanics 
of  the  movements  brought  about  by  the  contractions  of  skeletal 
muscle. 

A  skeletal,  or  voluntary  muscle,  is  enclosed  in  a  sheath  of 
connective  tissue,  or  muscular  fascia,  which  sends  in  at  places 
sheets  of  like  material  to  form  septa,  which  divide  the  muscle 
into  large  bundles  of  fibres.  Each  of  these  larger  bundles  is 
divided  by  thinner  sheets  of  intramuscular  connective  tissue 
into  fasciaili,  or  small  bundles,  and  again,  each  of  these  still 


»iii;i;uii;aiiiiiiE:;!!H?^ 


m- 


Fig.  40. — Sarcoiemma  of  mammalian 
muscle  highly  magnified. 

The  fibre  is  represented  at  a  place  where 
the  muscular  substance  has  become 
ruptured  and  has  shrunk  away,  leaving 
the  sarcoiemma  (with  a  nucleus  adher- 
ing to  it)  clear.  The  fibre  had  been 
treated  with  serum  acidulated  with 
acetic  acid. 


BiiiiiiiiiiiiHiniiiiHiii 

iiillllHliHIllHg 

iniiii:ii!:i|iiii  ii 

illlllilllllllllllBllil 

iiiiiiiinii;aiiii| 
iiiiiiiniiiiHi;!!^* 

iiliiii'i'ffllliiii 
[Illlliiljilililili 


ii:::in:ii  i  li 


nilliiiiii 


Fig.  41. — Muscular  fibre  of  a  mammal 
examined  fresh  in  serum,  highly  mag- 
nified, the  surface  of  the  fibre  being 
accurately  focussed. 

The  nuclei  are  seen  on  the  flat  at  the  sur- 
face of  the  fibre,  and  in  profile  at  the 
edges, 


contains  many  hundreds  of  the  fine  mt/scle  fibres  which  go  to 
make  up  the  chief  part  of  the  mass  of  the  muscle.  So  far  the 
structure  can  be  followed  with  the  naked  eye  (the  fasciculi  are 
the  grain  which  is  seen  in  boiled  flesh),  but  if  it  be  desired  to 
follow  the  structure  further,  and  see  the  ultimate  fibres  of  which 
each  fasciculus  is  made  up,  the  microscope  must  be  employed. 


The  Musctclav  System.  63 

In  the  fasciculus  the  constituent  fibres  lie  parallel  to  one  another,' 
each  is  about  3555  of  an  inch  in  diameter,  cylindrical,  or  nearly  so,  in 
section,  and  may  be  an  inch  or  more  in  length.  Each  fibre  has  a  sheath, 
called  the  sarcoleinma,  which  surrounds  and  incloses  the  contractile  sub- 
stance. This  sheath  is  so  thin  and  delicate  in  structure,  and  so  transparent, 
that  it  is  only  seen  at  places  where  in  the  process  of  preparation  the  con- 
tractile substance  within  it  has  been  broken  across  (see  Fig.  40).  At 
intervals  in  the  length  of  the  fibre,  oval  nuclei  are  to  be  seen  ;  these 
become  more  distinct  after  the  use  of  staining  agents. 

The  contractile  substance  ^  is  marked  off  into  short  dark  and  light  discs, 
so  as  to  give  a  striped  appearance  to  the  fibre.  When  the  fibre  contracts, 
the  light  bands  disappear,  and  the  dark  ones  swell  out.  It  is  probable 
that  the  protoplasm  forming  the  light  band  in  this  operation  passes  into  the 
dark  band.  The  consequence  is  that  the  distance  from  the  centre  of  one^ 
dark  band  to  the  centre  of  the  next  becomes  very  much  lessened,  and,  as 
this  takes  place  along  the  entire  length,  the  fibre  as  a  whole  becomes  much 
shorter  and  thicker. 

The  sarcolemma  passes  over  the  end  of  the  column  of  contractile  sub- 
stance at  each  end  of  the  fibre,  and  becomes  continuous  with  a  fine  thread 
of  fibrous  tissue  which  passes  to  form  a  constituent  part  of  the  tendoji  by 
which  the  muscle  is  attached  to  the  bone,  and  by  means  of  which  it  pulls 
upon  the  bone  when  it  contracts.  This  attachment  of  muscle  fibre  to  tendon 
is  best  seen  when  the  muscle  is  plunged  into  hot  water,  and  afterwards  a 
small  shred  from  the  junction  between  muscle  and  tendon  is  taken  and 
teased  and  examined  under  the  microscope.  The  hot  water  causes  the  end 
of  the  contractile  material  to  shrink  away  from  the  end  of  the  sarcolemma, 
as  seen  in  Fig.  42. 

The  vessels  which  carry  the  blood-supply,  and  the  nerve  carrying  the 
nerve-supply,  generally  enter  somewhere  near  the  middle  of  the  muscular 
mass.  The  artery  subdivides  into  branches  within  the  mass,  and  finally  the 
small  arterial  branches  (arterioles)  break  up  into  capillaries,  which  run  in 
oblong  meshes,  as  shown  in  Fig.  43,  with  the  long  branches  parallel  to  the 
length  of  the  muscle  fibres.  No  capillaries  ever  penetrate  any  of  the  muscle 
fibres ;  these  are  nourished  solely  by  diffusion  or  passage  m  sohctiofi,  in  both 
directions,  between  the  blood  within  the  capillary  and  the  muscular  sub- 
stance within  the  sarcolemma,  and  not  by  any  actual  contact  between  blood 


^  On  account  of  this  arrangement  o#  the  muscle  fibres,  a  shred  of  muscle 
teases  with  needles  more  easily  in  one  direction  than  another,  the  fibres 
being  more  easily  separated  than  broken  across.  The  same  is  true  of  nerve 
in  which  the  nerve  fibres  lie  side  by  side  in  a  similar  fashion.  To  observe 
the  minute  structure  of  either  kind  of  fibre,  a  small  shred  should  be  teased 
out  as  fine  as  possible,  by  means  of  two  needles  mounted  in  M-ooden  handles, 
and  then  observed  under  the  microscope. 

-  Only  an  outline  of  the  structure  of  the  contractile  substance  can  be 
given  here  ;  for  more  detailed  information  the  student  is  referred  to  Schafer's 
"Essentials  of  Histology." 


64. 


Elenientaj'y  Physiology. 


and  muscle  substance.  The  nutrient  materials  carried  to  the  muscle  fibres 
by  the  blood-stream  pass  out  in  solution  through  the  thin  wall  of  the 
capillary  vessel,  which  is  formed  by  a  single  layer  of  thin  flattened  cells 
tsee  p.  96).  The  fluid  so  separated  from  the  blood,  which  holds  in  solution 
the  materials  necessary  for  the  life  and  activity  of  the  muscle  fibres,  is  called 
the  lymph.    This  lymph  bathes  the  muscle  fibres,  and  the  nutrient  substances 


Fig.  42. — Termination  of  a  muscular  FiG.  43. — Capillary  vessels  of  muscle, 

fibre  in  tendon.     (Ranvier.) 

vt,  sarcolemma;  s,  the  same  membrane 
passing  over  the  end  of  the  fibre  ; 
/,  extremity  of  muscular  substance  ; 
c,  retracted  from  the  lower  end  of 
the  sarcolemma-tube  ;  t,  tendon- 
bundle  passing  to  be  fixed  to  the 
sarcolemma. 

which  it  contains  pass  through  the  sarcolemma  in  solution,  and  are  taken 
up  and  utilized  by  the  contractile  substance.  Besides  this  streaming  of 
nutrient  material  in  towards  the  muscle  substance,  there  is  also  a  stream  in 
the  opposite  direction  of  waste  material  away  from  the  muscle  substance  by 
means  also  of  the  intermediate  lymph  back  to  the  capillary  vessels. 

In  addition  to  this  stream  of  nutrient  substance,  which  has  been  in  the 


The  Mnsadai'  System,  65 

first  instance  formed  from  the  food  of  the  animal,  there  is  also  a  double 
stream  of  gas  in  solution  :  of  oxygen,  by  means  of  which  the  muscle  sub- 
stance is  able  to  decompose  or  oxidize  the  nutrient  matter  and  set  free  the 
store  of  energy  which  it  contains,  tozvards  the  muscle  substance  ;  and  of 
carho7i  dioxide,  a  chemical  product  of  this  oxidation,  and  hence  a  waste 
product,  aioay  from  the  muscle  substance  and  towards  the  capillaries,  Tlie 
oxygen  so  required  is  taken  in  at  the  lungs  during  respiration  in  a  way 
which  will  subsequently  be  described,  and  at  the  same  time  the  carbon 
dioxide  is  removed  from  the  blood  and  thrown  out  with  the  expired  air 
into  the  atmosphere  (see  p.  173). 

This  mode  of  exchange  between  the  blood  and  the  tissues,  whereby  the 
blood  supplies  nourishment  on  the  one  hand,  and  removes  waste  materials, 
the  products  of  chemical  change  going  on  in  the  tissues,  on  the  other,  is 
the  general  method  throughout  the  body.  The  cells  of  any  tissue  are  not 
bathed  in  blood,  but  in  lymph  which  has  exuded  through  the  thin  walls  of 
the  capillaries  ramifying  through  the  tissue.  An  artery  carrying  blood 
penetrates  the  part,  and  subdivides,  and  again  subdivides,  many  times  re- 
peated, so  giving  rise  to  a  large  number  of  very  fine  branches.  As  the  arterial 
branches  become  more  minute  at  each  subdivision,  their  walls  also  become 
thinner.  Finally,  they  merge  into  capillaries,  which  are  very  fine  tubes 
with  a  diavieter  oJ\^  to  3—  of  an  inch,  and  with  a  wall  of  extreme  thinness, 
consisting  of  a  single  layer  of  pavement  epithelium — that  is,  of  thin  flattened 
cells  joined  edge  to  edge.^  Through  this  thin  capillary  wall  the  lymph 
exudes  and  bathes  the  cells  which  are  to  be  fed  by  the  blood-stream. 

The  excess  of  lymph  which  collects  between  the  capillaries  and  cells  is 
gathered  up  by  a  different  system  of  capillaries,  called  lymphatic  capillaries, 
which  by  uniting  together  form  larger  lymphatic  vessels  (see  Fig.  45). 
These  larger  lymphatics  again  unite,  and  finally  all  the  lymph  so  collected 
is  carried  by  two  main  trunk  vessels  and  poured  into  two  large  veins  of  the 
neck,  so  returning  again  to  the  blood-stream.  The  lymphatic  trunk  on  the 
right  side  of  the  body  is  by  far  the  larger  of  these  two,  and  is  known  as 
the  thoracic  duct{'s,QQ  Fig.  44).  There  is  no  propelling  agent  corresponding 
to  the  heart  in  the  lymphatic  system  of  man,  but  the  lymphatics  are  closely 
beset  with  valves,  which  all  open  in  the  direction  of  the  thoracic  duct, 
and  hence  any  compression  of  the  lymphatics  by  muscular  movements 
always  determines  an  onward  flow  of  lymph  towards  the  thoracic  duct,  and 
so  back  to  the  circulating  blood. 

The  nerve  which  enters  the  muscle  consists  of  an  immense  number  of 
very  fine  long  fibres,  which  course  p^allel  to  one  another  in  the  nerve. 
Within  the  muscle  the  nerve  subdivides  in  the  same  manner  as  the  artery. 
At  each  division  the  number  of  nerve  fibres  in  each  branch  decreases,  for 
the  individual  fibres  do  not  divide,  and  so  at  length  there  are  within  the 
muscle  a  large  number  of  minute  nerve  twigs,  each  containing  a  number  of 
nerve  fibres.     Each  nerve  twig  supplies  a  number  of  muscle  fibres.     The 

^  See  p.  96. 


./-. 


,^11"! 


14 


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c  ■?;:  o 


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o 


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■■S"^-"  "5  bJOiH.^  i; 
rt        o  tv      ^.O 

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V;hoqCrt2'^>. 

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rt      0,^  b  s-a  3 

c +^         <"  J',  a 


o  'Ti       S  S  •*      4» 

•g  So.s.s.s  «-§  ^ 
o-.a ^i.o.^'S  rt 

:3 


-c  g  o 


The  Mtiscular  System. 


&7 


nerve  fibres  in  a  twig  finally  part  com- 
pany, and  each  fibre  usually  branches 
into  two  ;  these  branches  each  pierce 
the  sarcolemma  of  a  muscle  fibre  near 
the  middle,  and  end  in  a  small  oval- 
shaped  plate  of  coarsely  granular  proto- 
plasm, which  rests  on  the  contractile 
substance,  and  is  termed  an  e7id  plate. 
The  nerve  impulse  arrives  at  this  end 
plate,  and  in  some  manner  acts  on  the 
contractile  substance  of  the  fibre,  and 
causes  it  to  contract.  It  must  not  be 
assumed,  however,  that  all  the  fibres  in 
the  nerve  which  entered  the  muscle  ter- 
minate in  this  way  upon  the  contractile 
substance  of  the  muscle  fibres.  A  great 
number  of  them  do  so  ;  but  some  go  to 
the  coats  of  the  arterial  branches,  and 
regulate  the  blood-supply  to  the  muscle 
in  the  manner  above  described ;  others 
are  sensory  or  afferent  fibres,  and  instead 
of  carrying  nerve  impulses  to  the  muscle, 
carry  impulses  away  from  the  muscle  to 
the  central  nervous  system. 

Cardiac  muscle  fibres  are  also  trans- 
versely striated,  but  the  striations  are 
much  less  distinct.  There  is  no  sarco- 
lemma inclosing  the  fibre ;  the  fibres 
branch  and  the  branches  unite  with  those 
from  neighbouring  fibres,  and  the  nuclei 
lie  within  the  contractile  substance,  and 
not  upon  its  surface.  Each  fibre  con- 
sists of  a  series  of  short  cylindrical  cells 
united  end  to  end,  and  the  nuclei  of  the 
fibre  are  the  nuclei  of  these  cells  (see 
Figs.  46  and  47). 

Involuntary  or  plain  muscular  tissue 
differs   structurally  from  voluntary  and 
cardiac  muscle  in  that  it  does  not  con-^ 
sist  of  fibres  formed  each  by  the  union 

Fig.  45. — Superficial  lymphatics  of  the  breast,  shoulder,  and  I'pper  limb,  from  before 
(after  Mascagni  and  Sappey). 
a,  placed  on  the  clavicle,  points  to  the  external  jugular  vein  ;  b,  cephalic  vein  ;  c,  basilic 
vein ;  d,  radial  ;  e,  median  ;  /,  ulnar  vein  ;  g;  great  pectoral  muscle,  cut  and  turned 
outwards  ;  i,  superficial  lymphatic  vessels  and  glands  above  the  clavicle  ;  2,  infra- 
clavicular glands  ;  3.  3,  pectoral  glands  ;  4,  4,  axillary  glands  ;  5,  two  small  glands 
placed  near  the  bend  of  the  arm  ;  6,  radial  lymphatic  vessels  ;  7,  ulnar  lymphatic 
vessels  ;  3,  8,  palmar  lymphatics. 


68 


Elementary  Physiology. 


of  a  large  number  of  cells,  but  of  single  cells.  These  cells  are  long 
and  fusiform  in  shape  (Fig.  48),  with  an  oval  or  rod-shaped  nucleus.^ 
The  cells  are  longitudinally  striated,  but  show  no  cross  striation  (compare 
Figs.  41  and  48)  ;  they  are  united  together  by  a  small  amount  of  connective 
tissue,  and  each  is  surrounded  by  a  delicate  sheath,  which  is,  however, 
invisible  unless  the  contractile  material  has  been  broken  across. 


Fig.  46. — Muscular  'fibres  from  the  heart, 
magnified,  showing  their  cross-striae, 
divisions,!  and  junctions.  (Schweig- 
ger-Seidel.) 

The  nuclei  and  cell-junctions  are  only 
represented  on  the  right-hand  side  of 
the  figure. 


Fjg.  47. — Six  muscular  fibre  cells 
from  the  heart.  (Magnified  425 
diameters. ) 

a,  line  of  junction  between  two  cells  ; 
3,  c,  branching  of  cells.  (From  a 
drawing  by  J.  E.  Neale.) 


The  different  kinds  of  muscular  tissue  also  vary  in  the 
manner  of  their  response  on  excitation.  Under  normal  con- 
ditions in  the  body,  skeletal  muscle  never  contracts  except 
when  excited  to  do  so  by  nerve  impulses,  which  travel  to  it 
along  the  motor  (efferent)  nerve  fibres.  The  involuntary 
muscle  fibres  which  line  the  intestine,  and  those  lying  in  the 
substance  of  the  spleen,  on  the  other  hand,  continue  to  contract 
rhythmically  even  after  they  have  been  severed  from  all  their 
nerve  connections.  The  same  is  true  of  heart  muscle,  for,  not 
only  will  the  entire  heart  of  a  cold-blooded  animal,  such  as  the 

^  An  oval  nucleus  is  seen  in  the  involuntary  muscle  cells  of  the  muscular 
coats  of  the  small  intestine,  and  a  rod-shaped  nucleus  in  the  muscular  coats 
of  the  larp-er  arteries. 


The  Muscidar  System. 


69 


frog,  continue  to  beat  for  many  hours 
after  removal  from  the  body,  but  even 
a  strip  of  the  muscular  tissue,  cut  out 
from  the  heart,  and  containing  no  ner- 
vous mechanism,  will  beat  for  a  long 
time.^  The  mammalian^  heart  ceases 
to  beat  soon  after  removal  from  the 
body,  but  this  is  due  to  the  more  rapid 
death  of  the  muscle  cells,  caused  by  the 
stoppage  of  the  blood-circulation  through 
them;  for,  when  means  are  taken  to 
continue  the  circulation  of  the  blood, 
even  the  mammalian  heart  continues 
to  beat. 

When  a  skeletal  muscle  and  the 
nerve  passing  to  it  are  removed  from 
the  body,^  the  muscle  can  be  caused  to 
contract  by  stimulating  its  nerve  in 
various  ways,  such  as  (mechanically)  by 
pinching  or  hammering  it,  (chemically) 
by  applying  a  crystal  of  salt  to  it,  or 
(electrically)  by  the  action  of  an  electric 
current. 

Electric  stimulation  is  the  most  suitable 
form  for  experimental  purposes,  because  the 
nerve  is  not  injured  thereby,  and  the  excitation 
may  be  many  times  repeated.  By  a  certain 
arrangement  of  apparatus  a  graphic  record  of 


^  The  heart  has,  in  its  substance,  small  groups 
of  nerve  cells  called  ganglia,  and  the  purpose 
of  the  strip  experiment  is  to  show  that  the  rhyth- 
mic beating  is  not  due  to  these  nerve  cells,  but 
is  an  inherent  property  of  cardiac  muscle,  the 
strip  being  cut  out  so  as  not  to  include  any 
nerve  cells, 

-  The  mammalia  are  that  class  of  animals 
which  suckle  their  young, 

^  These  experiments  can  best  be  performed 
with  a  muscle  and  nerve  taken  from  a  cold- 
blooded animal  (frog),  for  a  mammalian  muscle 
dies  very  soon  after  its  blood-supply  is  stopped. 


Fig.  48. — Muscular  fibre  cells 
from  the  muscular  coat  of 
the  small  intestine,  highly 
magnified. 

A,  a  complete  cell,  showing  the 
nucleus  with  intra-nuclear 
network,  and  the  longitu- 
dinal fibrillation  of  the  cell 
suhstanC3,with  finely  vacuo- 
lated protoplasm  between 
the  fibrils  ;  B,  a  cell  broken 
in  the  process  of  isolation  ; 
a  delicate  enveloping  mem- 
brane projects  at  the  broken 
end  a  little  beyond  the  sub- 
Stance  of  the  cell. 


70 


Elementary  Physiology. 


the  response  of  a  muscle  to  electrical  stimulation  may  be  obtained.^  In- 
duced currents  from  a  secondary  coil  are  made  use  of,  and  these  are 
conveyed  to  the  nerve  by  two  wires,  called  electrodes,  placed  parallel  to 
each  other  and  about  -j^  of  an  inch  apart.  The  electrodes  are  joined  up 
to  the  terminals  of  the  secondary  coil,  and  the  nerve  laid  across  them. 
On  making  or  breaking  the  electric  current  in  the  primary  circuit,  a  tran- 
sitory current  is  induced  in  the  secondary  circuit,  which  excites  the  nerve 
and  starts  a  nerve  impulse,  and  this  in  turn  excites  the  muscle  to  con- 
tract. •  The  muscle  is  fixed  at  one  end,  and  its  other  end  is  attached  by  a 
thread  to  the  short  end  of  a  lever.     When  the  muscle  contracts  the  lever 


Fig.  49. — Simple  muscle  curve. 

The  nerve  attached  to  the  muscle  was  stimulated  at  a,  and  at  b  the  muscle  began  to 
contract.  The  period  occupied  by  the  interval  between  a  and  b  is  termed  the 
"  latent  period."  The  lower  tracing,  taken  by  a  writing-point  attached  to  a  vibrating 
tuning-fork,  shows  j^o  second  intervals  ;  the  small  undulations  upon  it  are  due  to 
overtones. 

moves,  and,  by  means  of  a  writing-point  attached  to  its  long  end,  traces 
a  curve  on  a  smoked  surface  of  paper  which  is  caused  to  move  past  the 
writing-point. 

When  a  single  electric  stimulu?  is  applied  to  the  nerve,  the  tracing 
obtained  of  the  muscular  contraction  is  similar  to  that  shown  in  Fig.  49. 
There  is  first  a  very  short  pause  before  the  contraction  begins,  called  the 
latent  period y  then  the  contraction  begins,  and  is  followed  by  the  relaxation. 
The  relative  times  occupied  by  the  periods  of  contraction  and  relaxation 
vary  with  the  load  against  which  the  muscle  contracts ;  but  the  total  period 
of  contraction  and  relaxation  is  fairly  constant,  and  amounts  approximately 
to  -/g  second.  If  now  the  stimulation  be  given  rhythmically  at  regular 
periods,  the  effect  produced  will  vary  according  to  the  rapidity  with  which 
the  stimuli  or  shocks  succeed  one  another.     If  a  small  number  of  stimuli 


^  For  details  of  the  methods  of  obtaining  graphic  records  of  muscle  con- 
tractions, see  Brodie,  "  Essentials  of  Experimental  Physiology." 


TJie  Muscitlar  System.  J\ 

(4  to  6)  per  second  be  applied,  each  conliaction  has  time  to  pass  otif  before 
the  succeeding  one  is  evoked,  and  the  result  is  a  series  of  separate  con- 
tractions or  twitches.  If  the  rate  of  stimulation  be  more  rapid,  each 
stimulus  is  applied  before  the  contraction  due  to  the  preceding  one  has 
passed,  off,  and  there  is  a  summation  of  effect ;  the  muscle  never  becomes 
completely  relaxed,  but  a  sinuous  line  is  traced,  due  to  the  muscle  being 
more  or  less  contracted  the  whole  time,  the  sinuosities  being  caused  by  the 
individual  stimuli.  With  a  still  more  rapid  rate  of  30  to  40  stimuli  per 
second  these  sinuosities  disappear,  and  the  muscle  remains  permanently 
contracted.  In  this  condition  each  stimulus  is  thrown  in  at  the  height  of 
the  contraction  due  to  the  preceding  stimulus,  and  so  a  maximum  con- 
traction is  maintained.  This  permanent  contraction  is  known  as  coiiip  'cle 
telanus,  the  imperfect  fusion  with  a  slower  rate  being  termed  incomplete 
tetaujis.  The  ordinary"  natural  contractions  of  the  skeletal  muscles  in  the 
body  are  incomplete  tetani  ;  the  rate  at  which  the  natural  nerve  impulses 
are  sent  to  the  muscles  being  about  12  to  14  per  second.  A  muscle  cannot 
be  maintained  in  a  tetanized  condition  for  any  considerable  time,  because  it 
becomes  fatigued  and  gradually  relaxes,  although  the  nerve  does  not  become 
fatigued  and  the  impulses  are  still  conveyed  along  it.  Even  single  stimuli 
repeated  at  somewhat  longer  intervals  than  ^\j  of  a  second  are  sufficient  to 
fatigue  a  muscle  when  the  stimulation  is  long  continued. 

Both  cardiac  muscle  and  involuntary  muscle  have  a  much  longer  latent 
period  than  voluntary  or  skeletal  muscle.  This  may  be  shown,  in  the  case 
of  involuntary  muscle,  by  exposing  the  intestine  in  an  animal  which  has 
just  been  killed,  and  directly  stimulating  it  by  pricking  ^^ith  a  sharp  point. 
The  intestine  contracts  at  the  point  touched,  but  only  after  an  obvious  delay 
apparent  to  the  eye.^ 

Cardiac  mttscle  further  differs  from  voluntary  muscle,  in  that 
it  cannot  be  tetanized.  The  muscle  fibres  possess  naturally  the 
property  of  contracting  at  regular  intervals.  Just  after  each 
contraction,  the  fibres  pass  into  a  refractory  condition,  and 
cannot  by  any  stimulation  be  caused  immediately  to  contract. 
If  this  tendency  to  rhythmic  contraction  has  by  any  means 
been  so  much  weakened  that  the  contractions  do  not  take 
place  spontaneously,  then  stimulation  may  reinforce  it,  and 
cause  contractions  at  intervals ;  but  by  no  means  can  rapidly 
repeated  rhythmic  stimuli  be  summated,  and  the  heart  muscle 
sent  into  tetanus.  After  a  contraction  has  taken  place,  the 
fibres  become  inexcitable  for  a  brief  period,  and  relax,  in  spite 
of  stimulation.     The  rate  at  which  the  contractions  take  place 

^  The  latent  period  of  voluntary  muscle  is  much  too  short  to  be  appreciated 
in  such  a  manner. 


J 2  Elementary  Physiology. 

can,  however,  be  altered  by  stimulation,  either  of  the  heart 
muscle  directly,  or  through  its  nerves. 

There  are  two  nerves  which  carry  impulses  to  the  heart, 
and  alter  its  rhythm  {i.e.  rate  of  beating) ;  these  nerves  are 
branches,  respectively,  from  a  nerve  called  the  vagus,  and  from 
a  chain  of  nerves  called  the  sympathetic.  The  vagus  branch 
slows  the  heart  when  stimulated,^  and  if  sufficiently  strongly 
excited  stops  it  for  a  time,  but  only  for  a  time ;  afterwards 
the  inherent  property  of  contracting  which  the  cardiac  muscle 
fibres  possess  asserts  itself,  and  no  matter  how  strong  the 
stimulus,  the  accumulated  tendency  to  contract  becomes  too 
strong  for  the  inhibiting  ^  stimulus,  and  the  heart  recommences 
to  beat  slowly. 

The  sympathetic  branch,  on  the  other  hand,  increases  the 
rhythm  when  it  sends  impulses  to  the  heart,  causing  the  heart 
to  beat  much  more  rapidly.'^ 

The  cardiac  muscle  fibres  then  possess  the  property  of 
rhythmic  contractility,  and  retain  this  property  even  when  cut 
off  from  all  nerve  mechanism ;  and,  further,  stimulation 
through  nerves,  or  otherwise,  can  only  alter  the  rate  of  this 
rhythmic  contraction,  and  not  altogether  remove  it,  either  by 
keeping  the  fibres  permanently  contracted  or  permanently 
relaxed. 

This  property  of  rhythmic  contractility  is  shared  to  a  less 
perfect  degree  by  the  involuntary  muscle  fibres  of  the  spleen 
and  intestine,  but  is  not  possessed  at  all  by  voluntary  muscle, 
which  only  contracts  when  stimulated  to  do  so.  The  normal 
or  natural  excitation  to  contraction  of  voluntary  muscle  is 
given  by  nerve  impulses,  which  are  sent  out  from  the  central 
nervous  system  at  a  rate  of  lo  to  12  per  second,  and  cause 
the  muscle  to  pass  into  incomplete  tetanus. 


'  This  effect  may  be  shown  experimentally  by  cutting  these  nerves, 
and  then  stimulating,  in  each  case,  the  end  next  to  the  heart. 

2  Inhibition  means  the  stoppage  of  any  normal  action  by  nervous 
mechanism  ;  the  stoppage  of  the  heart,  or  its  slowing,  by  the  vagus  as 
described  above  is  an  example.  In  consequence  of  this  action  the  vagus 
is  said  to  be  the  inhibitory  nci've  of  the  heart. 

^  This  may  be  shown  by  stimulating  the  cardiac  branch  of  the  sym- 
pathetic. 


TJie  Muscular  System  73 

A  single  skeletal  muscle  rarely  or  never  contracts  alone  in  the  manner 
described  above,  but  always  in  consort  with  a  number  of  other  muscles. 
For  even  the  simplest  movements  require  the  combined  action  of  several 
muscles  to  carry  them  out.  Also,  if  the  movement  is  at  all  complicated, 
the  different  muscles  involved  in  it  must  contract  in  a  definite  order  and 
time  with  regard  to  one  another,  and  with  a  definite  strength  of  contraction. 
This  harmonious  working  of  the  muscles  together  is  spoken  of  as  co-ordina- 
tion. Unless  the  co-ordination  be  perfect  the  movement  will  be  performed 
in  a  clumsy  and  imperfect  way.  Complicated  movements  requiring  much 
co-ordination,  such  as  walking,  talking,  and  grasping  and  handling  objects, 
are  learnt  by  practice  by  the  infant  just  in  the  same  manner  as  other  skilled 
movements,  such  as  writing,  piano-playing,  cycle-riding,  rowing,  swimming, 
and  countless  other  such  accomplishments  are  learnt  later  in  life.  When 
once  any  such  complicated  movement  has  been  learnt  it  becomes  automatic, 
the  will  is  only  concerned  in  starting  or  stopping  the  cycle  of  muscular 
contractions,  and  the  various  movements  are  carried  out  in  perfect  co-ordina- 
tion or  rotation  by  lower  nerve  centres,  without  the  attention  being  con- 
sciously fixed  upon  them.  If  certain  parts  of  the  nervous  system  be  injured, 
however,  this  co-ordination  is  interfered  with,  some  of  the  paths  of  nerve 
discharge  become  blocked,  and  the  process  has  to  be  learnt  again  ;  if, 
indeed,  the  injury  has  not  been  such  as  to  block  all  possible  paths  for  the 
carrying  out  of  the  necessary  movements.  For  example,  an  injury  to  a 
certain  part  of  the  brain  may  affect  the  speech,  so  that  certain  words  cannot 
be  said  at  will,  because  the  injury  has  blocked  the  track  of  communication 
between  certain  nerve  cells  in  the  brain  and  the  muscles  of  the  tongue  and 
lips,  of  which  the  movement  is  necessary  for  the  production  of  these  words. 
Such  words  may  be  acquired  again  by  the  establishment  of  communication 
through  some  more  roundabout  route,  as  the  result  of  practice  or  repeated 
trial.  This  is  but  one  example.  In  nearly  all  cases  where  any  part  of  the 
brain  is  injured,  "^t  paralysis  or  loss  of  function  which  at  first  appears  as  a 
result  gradually  vanishes,  even  though  the  injury  remain  permanently, 
because  other  parts  of  the  brain  take  on  the  work  of  the  injured  part. 
There  is  a  free  choice  of  paths  along  which  nerve  impulses  may  pass  from 
the  central  nervous  system  to  the  muscles,  and  the  usual  one  is  merely  the 
easiest,  most  convenient  and  most  used  one.  If  this  is  shut  off,  a  new  one 
is  soon  discovered.  Just  as  with  our  present  intricate  meshwork  of  tele- 
graphic communication  over  the  world.  When  communication  is  interrupted 
between  two  important  places,  there  is  only  a  temporary  hitch,  for  soon 
it  is  discovered  which  is  the  next  easiest  line  of  connection  between  the  two 
places. 

The  Mechanics  of  the  Skeletal  Movements. 

The  bones  form  levers  which  the  muscles  attached  to  them 
are  capable  of  moving.     A   lever  is  a  rigid  bar  capable  of 


74 

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Elementary  Physiology. 

fixed  point  called  \}i\&  fulcrum  under  the  action 
g  of  forces  applied  along  its  length  at 
b  different  points.  Any  number  of 
";;  u  forces  may  act  on  the  same  lever, 
but  usually  the  simplest  case  only  is 
considered  of  two  forces  acting  in 
opposition  and  balanced  by  their 
resultant  acting  at  the  fulcrum.  One 
of  these  forces  is  usually  the  pull  of 
the  earth  on  a  body,  and  is  known 
as  the  weiij^ht  (W),  while  the  op- 
posing force  is  called  the  power  (P). 


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It  is  usual  to  divide  levers  into  three 
orders  or  classes,  according  to  the  dispo- 
sition with  regard  to  one  another  of  fulcrum, 
power,  and  weight ;  but  this  division  is  an 
arbitrary  one,  for  the  principle  is  the  same 
in  all  three  orders.  The  three  orders  are 
figured  in  the  accompanying  sketch  ;  in  the 
first  order  the  fulcrum  is  in  the  middle, 
and  the  weight  and  power  at  opposite  sides  ; 
in  the  second,  the  fulcrum  is  at  one  end,  and 
the  power  at  the  other,  the  weight  being  in 
the  middle  ;  while  in  the  third,  the  fulcrum 
is  still  at  one  end,  but  the  power  is  in  the 
isiddle  and  the  weight  at  the  other  end. 

When  the  power  and  weight  balance 
each  other,  there  exists  a  simple  relationship 
between  their  magnitude,  each  being  in- 
versely  proportional  to  its  distance  from  the 
fulcrum.  That  is  to  say,  if  P,  for  example, 
is  three  times  as  far  from  F  as  W  is,  it  will 
only  require  to  be  one- third  as  great  to 
balance  it.  If  P  exceed  by  a  little  this 
amount,  the  weight  (W)  v/ill  be  raised  while 
P  descends ;  but  the  matter  will  be  equalized 
in  this  way,  that  P  will  go  down  three  times 
as  far  as  W goes  zip. 

The  ratio  of  W  to  P  is  called  the  mecha- 
nical advantage.  This  can  have  any  value  in 
the  first  order,  but  must  be  greater  than 
unity  in  the  second,  and  less  than  unity  in 


The  Muscular  System.  75 

the  third.'  The  fact  that  P  can  be  made  much  less  than  W  is  an  advantage 
ill  one  respect  only,  viz.  that  a  small  force  can  be  made  to  lift  a  large  weight, 
but  in  order  to  do  so  the  small  force  must  move  through  a  correspondingly 
greater  distance.  The  force  multiplied  by  the  distance  is  called  the  ivork 
done,  and  in  all  cases  the  work  done  by  P  equals  that  done  on  W. 

In  the  body  it  is  usually  less  of  an  advantage  to  have  a 
small  force  acting  through  a  long  distance  than  to  have  a 
large  force  acting  for  a  small  distance.  The  muscles  and  their 
tendons  are  very  strong,  and  the  tendons  are  usually  inserted 
closer  to  the  fulcrum  than  the  weight  or  load  to  be  raised ;  as, 
for  example,  in  raising  the  forearm.  Hence  a  large  range  of 
movement  is  obtained  with  a  small  amount  of  contraction  of 
the  muscle,  to  pay  for  which  advantage  the  pull  of  the  muscle 
through  its  tendon  must  be  many  times  greater  than  the  weight 
to  be  raised.  If  the  biceps  muscle  were  attached  halfway  out 
along  the  radius  instead  of  near  the  elbow  joint,  the  amount  of 
contraction  of  its  fibres  in  order  to  bring  the  forearm  from  the 
extended  to.  the  flexed  position  would  be  enormous,  and,  in 
fact,  impossible  for  muscle  fibres  as  constituted  in  the  body. 
Hence  the  insertion  is  near  the  elbow  joint,  and  a  smaller  and 
correspondingly  more  powerful  pull  on  the  tendon  accomplishes 
the  purpose. 

Examples  in  the  body  of  levers  of  the  first  order  are  the 
movements  of  the  head  on  the  atlas,  and  of  the  trunk  at  the 
hip  joints. 

A  good  example  of  the  second  is  raising  the  body  on 
tiptoe ;  here  the  toes  and  metatarsal  bones  lie  at  the  fulcrum, 
the  weight  is  at  the  ankle  joint,  and  the  power  is  applied 
through  the  tendons  of  the  calf  muscles  at  the  back. 

The  third  class  is  by  far  the  commonest  in  the  movements 
of  the  body;  it  is  seen  in  the  movements  at  the  jaw,  in  flexion 
of  the  elbow  and  knee  joints,  and  in  many  movements  of  the 
other  joints. 

This  classification  of  levers  has  been  given  here  in 
deference  to  established  custom,  but  it  should  be  remembered 
that   there   is   no    essential   difference   between   the   different 

^  The  second  and  third  orders  interchange  when  it  is  equal  to  unity,  for 
then  P  and  W  are  applied  at  the  same  point  of  the  lever. 


J 6  Elementary  Physiology, 

orders,  and  that  the  classification  has  no  importance  save 
as  a  means  of  description.  The  essential  feature  in  the  lever 
is  that  the  force  may  be  diminished  if  its  leverage  is  in- 
creased. But  this  involves  increased  movement,  and  although 
for  many  mechanical  purposes  this  may  be  advantageous,  in 
the  body  it  is  a  decided  disadvantage,  and  hence  extent  of 
contraction  is  economized  by  increasing  the  applied  force.  It 
should  also  be  remembered  that  in  the  body  the  tendons  do 
not  pull  parallel  to  the  weight  in  many  cases,  and  hence  the 
pull  on  the  tendons  is  still  further  increased.  It  follows  that 
the  tendons  of  muscles  must  be  exceedingly  tough  and  strong 
structures,  and  this  is  actually  the  case,  the  larger  tendons 
being  capable  of  standing  without  breaking  a  pull  many  times 
greater  than  the  weight  of  the  body. 

The  erect  position  of  the  body  can  only  be  maintained 
when  the  vertical  line  through  the  centre  of  gravity  falls  within 
an  area  drawn  to  include  the  soles  of  the  feet ;  but  this  is  not 
all  that  is  necessary  to  the  maintenance  of  the  erect  posture. 
The  joints  must  be  stiffened  by  a  balancing  of  forces  due  to 
a  definite  amount  of  tonic  contraction  ^  in  the  muscles  situated 
before  and  behind  them.  This  balancing  of  forces  is  learnt 
in  infancy,  and  afterwards  is  always  maintained,  when  we 
assume  the  erect  position,  without  conscious  effort  or  attention. 
The  activity  of  the  nerve  centres  is  necessary  for  the  main- 
tenance of  the  erect  position;  hence  a  shock  to  the  nervous 
system,  say  by  a  blow  on  the  head,  or  a  sudden  fright,  causes 
the  person  to  fall  down  in  a  heap.  The  nervous  impulses 
which  kept  the  various  sets  of  muscles  tonically  contracted 
are  stopped,  the  muscles  relax,  the  body  is  in  a  position  of 
unstable  equilibrium,  bending  at  the  joints  takes  place,  and 
the  person  falls  to  the  ground. 

The  same  thing  is  seen  in  nodding  of  the  head  as  a  person 
falls  asleep  in  a  sitting  posture ;  the  muscles  at  the  front  and 

^  The  term  "tonic  contraction  "  means  a  certain  amount  of  passive  or 
continuous  contraction  that  a  muscle  is  kept  in  under  given  conditions.  A 
muscle,  even  when  not  undergoing  active  contraction,  is  never  completely 
relaxed,  but  the  tendon  is  kept  taut  and  ready  for  action,  so  that  there  is  no 
slack  in  the  tendon.  The  same  is  true  of  involuntary  fibres  surrounding 
blood-vessels,  etc.,  which  are  always  in  a  more  or  less  contracted  condition. 


The  Muscular  System. 


77 


U 


back  of  the  neck,  the  tonic  contractions  of  which  had  previously 
balanced  one  another,  become  relaxed,  and  the  head  usually 
falls  forward,  because  the  mass  of  the  head  in  the  normal 
sitting  position  lies  more  in  front  of  the  articulation  between 
skull  and  atlas  than  behind  it.  Sound  sleep 
in  a  standing  position,  or  walking  during 
sound  sleep,  are  impossible  because  of  this 
relaxation  of  the  muscles  through  inhibition 
of  the  tonic  constricting  nerve  impulses  to 
them.  In  cases  where  people  walk  or  talk 
in  their  sleep  the  nervous  system  is  not  at 
rest  to  the  normal  amount  of  sound  sleep. 
The  same  is  true  in  the  case  of  dreaming ; 
portions  of  the  brain  which  ought  to  be  in 
a  resting  condition  are  active.  Even  in 
the  soundest  sleep,  however,  there  is  not 
complete  quiescence  of  the  entire  nervous 
system;  the  respiration  must  be  kept  up 
throughout  by  the  activity  of  certain  nerve 
cells,  situated  in  a  part  of  the  central  ner- 
vous system  known  as  the  medulla  oblon- 
gata (see  p.  223),  which  rhythmically  send 
out  impulses  to  the  respiratory  muscles  and 
cause  these  to  contract.  The  rhythm  of 
the  heart  is  also  probably  regulated  during 
sleep  by  the  nerves  which  pass  to  it ;  but, 
generally  speaking,  nervous  and  muscular 
activity  are  reduced  to  a  minimum,  in  order 
to  allow  these  two  tissues,  which  are  con- 
tinually in  action  during  the  waking  hours, 
to  become  refreshed  and  recuperated  by 
the  nutrient  blood-stream,  which  carries 
them  fresh  supplies  and  removes  their  waste 
products. 

The  muscles  which  by  their  balanced 
tonicity  support  the  body  in  the  standing 
position  are  diagrammatically  shown  in  Fig.  51.     The  muscles 
in   certain  positions  are  aided  by  ligaments  which   come  on 


Fig.  51. — Diagram  showing 
the  action  of  the  chief 
muscles  which  keep  the 
body  erect. 

The  arrows  show  the  direc- 
tion in  which  the  muscles 
pull.  Those  in  front  op- 
pose and  balance  those 
at  the  back. 


78  Elementary  Physiology. 

the  stretch  in  the  standing  position.  Thus,  at  the  knee,  the 
bones  are  completely  extended  by  the  extensor  muscles  of  the 
thigh,  and  motion  forward  is  prevented  by  the  ligaments  of 
knee  joint. 

The  reader  can  easily  analyze  roughly  those  muscular 
movements  which  take  place  in  the  acts  of  walking  and  running. 
When  a  person  starts  to  walk  from  a  standing  posture  the 
weight  is  first  thrown  on  one  foot  by  a  slight  movement  of  the 
body  to  that  side,  then  the  other  foot  is  raised  from  the  ground 
and  carried  forward  by  a  flexion  in  front  at  the  hip  joint  and 
a  slight  flexion  at  the  knee.  At  the  same  time,  the  other  foot 
is  raised  on  tiptoe  by  the  contraction  of  the  powerful  muscles 
of  the  calf  of  the  leg,  and  the  whole  body  is  swung  forward 
by  a  movement  at  the  ankle  and  hip  joints.  As  this  goes  on 
the  foot  which  was  raised  and  carried  forward  comes  on  the 
ground,  and  the  weight  is  gradually  received  on  it  as  the  body 
is  thrust  forward.  The  other  leg  is  next  brought  forward  by 
a  flexion  at  hip  and  knee,  and  swung  out  in  front  to  commence 
another  step. 

In  running,  the  muscular  contractions  are  more  vigorous, 
and  for  a  brief  period  during  each  stride  both  feet  are  off  the 
ground.  The  body  is  thrown  forward  just  before  each  foot 
leaves  the  ground  by  a  sudden  extension  of  the  leg  at  the  hip 
and  knee.  In  jumping  a  similar  sudden  extension,  but  of  both 
legs  at  once,  is  the  means  of  progression 


CHAPTER    IV. 

POSITION  OF    THE    VISCERA. 

The  great  cavity  of  the  trunk  is  'divided  into  two  compartments 
by  the  diaphragm.  The  upper  compartment  is  called  the 
thoracic  cavity,  or  thoi'ax^  and  the  lower  is  the  abdominal 
cavity,  or  abdo7Jien.  The  organs  contained  in  the  thorax  are 
termed  the  thoracic  viscera^  and  those  in  the  abdomen  the 
abdominal  viscera. 

The  Thoracic  Viscera.^ 

The  thorax  contains  the  heart  and  the  great  blood-vessels 
passing  to  and  from  it;  the  lungs  and  the  branches  of  the 
trachea^  or  windpipe,  called  the  hwichi.,  which  convey  the  air 
to  them ;  the  remnants  of  a  gland  called  the  thymus.,  which  is 
relatively  large  in  the  infant  but  becomes  gradually  insignificant 
in  size  towards  middle  life ;  part  of  the  (esophagus.,  which  is  a 
straight  muscular  tube  serving  to  convey  the  food  from  the 
mouth  to  the  stomach ;  the  thoracic  duct.,  which  is  a  vessel  for 
conveying  the  lymph  (see  p.  65)  to  the  position  where  it  joins 
the  blood-stream  by  opening  into  a  vein  in  the  neck ;  and 
various  nerves  passing  either  to  these  organs  or  on  to  the 
diaphragm  and  the  abdominal  viscera. 

The  heat't  is  shaped  like  a  cone  with  rounded  apex  and 
base,  and  is  about  the  same  size  as  the  closed  fist.  It  lies  in 
the  anterior  part  of  the  thorax  a  little  more  to  the  left  of  the 
median  line  than  to  the  right  (see  Fig.  53).  The  apex  lies 
opposite  to  a  point  about  an  inch  and  a  half  below  the  left 

'  The  student  is  recommended  to  accompany  this  description  with  the 
dissection  of  an  animal. 


8o 


Elementary  Physiology 


nipple,  and  three  inches  from  the  middle  line.  Here  the  apex  of 
the  heart  touches  the  thoracic  wall  as  it  is  thrown  forward  by 
each  beat,  and  these  beats  may  be  distinctly  felt,  and  in  some 
cases  seen  through  the  skin  at  this  point  {the  apex  beat). 
Feeling  the  heart  beat  here  has  given  rise  to  a  popular 
impression  that  the  heart  lies  much  more  to  the  left  side  than 


Fig.  52. — The  lower  half  of  the  thorax,  with  four  lumbar  vertebrae,  showing  the 
diaphragm  from  before.  (Allen  Thomson,  after  Luschka.)  \ 
a,  sixth  dorsal  vertebra  ;  b,  fourth  lumbar  vertebra  ;  c,  ensiform  process  ;  d,  </,  aorta, 
passing  through  its  opening  in  the  diaphragm  ;  e,  oesophagus ;  y,  opening  in  the 
tendon  of  the  diaphragm  for  the  inferior  vena  cava  ;  i,  central,  2,  right,  and  3,  left 
division  of  the  trefoil  tendon  of  the  diaphragm  ;  4,  right,  and  5,  left  costal  part, 
ascending  from  the  ribs  to  the  margins  of  the  tendon;  6,  right,  and  7,  left  crus ; 
8,  to  8,  on  the  right  side,  the  sixth,  seventh,  and  eighth  internal  intercostal  muscles, 
deficient  towards  the  vertebral  column,  where  in  the  two  upper  spaces  the  levatores 
costarum  and  the  external  intercostal  muscles  9,  9,  are  seen  ;  10,  10,  on  the  left  side, 
subcostal  muscles. 

is  really  the  case ;  for  from  this  point  the  heart  is  directed 
upwards,  and  to  the  right,  the  base  lying  above  and  to  the 
right,  so  that  a  small  part  of  the  heart  lies  to  the  right  side  of 
the  sternum  as  shown  in  Fig.  53.     The  great  vessels  which 


Position  of  the   Viscera.  8i 

carry  the  blood  to  and  from  the  four  chambers  of  the  heart  all 
enter  and  leave  it  in  a  group  at  the  base  (see  Fig.  54,  p.  83). 
k.  large  vein  called  the  infenor  vena  cava  carries  the  greater 
part  of  the  blood  returning  to  the  heart  from  the  lower  part  of 
the  body.  This  vessel  penetrates  the  diaphragm  at  the  back 
near  the  vertebral  column  (see/,  Fig.  52),  and  passes  up  parallel 


Fjg.  53. — Transverse  section  through  the  thorax. 
The  section  is  carried  above  the  heart,  but  below  the  division  of  the  trachea. 
I,   sternum;   2,  body  of  dorsal  vertebra;    3,  spinous  Drocess;   4,   spinal  canal;  5,  rib; 
6,  inner  layer  of  pleura  ;  7,  outer  layer  of  pleura ;  8,  pericardium ;  9,  right  bronchus  ; 
10,  left   bronchus;    11,   oesophagus;    12,    heart;    17,   aorta,  ascending;    14,    aorta, 
descending;  15,  left  lung;  16,  right  lung;   17,  pulmonary  arteries. 

and  close  to  the  vertebral  column  in  the  thorax  to  enter  the 
thin-walled  right  upper  chamber  of  the  heart  {the  right  auricle). 
The  blood  coming  from  the  head,  neck,  and  arms  is  collected 
into  another  great  vein  (the  supenor  vena  cava),  and  by  this  is 
also  discharged  into  the  right  auricle.  From  the  right  auricle 
the  blood  flows  into  the  right  ventricle,  being  assisted  in  its 
flow  towards  the  end  when  the  ventricle  is  nearly  full  by  the 
contraction  of  the  auricle,  which  distends  the  ventricle.^     The 

'  For  a  description  of  the  heart  and  its  valves,  see  the  anatomy  of  the 
circulatory  system,  p.  1 13;  an  outline  only  is  given  here  to  enable  the 
student  to  understand  the  arrangement  of  the  great  vessels  in  the  thorax. 

G 


82  Elementayy  Physiology. 

ventricle  next  contracts,  and  the  communication  between 
auricle  and  ventricle  being  closed  by  a  valve  opening  towards 
the  ventricle,  and  a  communication  being  opened  between  the 
ventricle  and  a  large  artery  in  connection  with  it  called  the 
pulmonary  artery  by  the  forcing  open  of  a  valve  directed 
towards  this  artery,  the  blood  is  discharged  under  pressure  from 
the  right  ventricle  into  the  pulmonary  artery.  The  pulmonary 
artery  passes  upwards  from  the  heart  and  soon  divides  (see 
Fig.  52)  into  two  branches,  one  of  which  passes  to  each  lung 
and  enters  at  the  root,  which  is  situated  on  the  mesial 
surface  of  the  lung  nearer  the  apex  than  the  base  (see  Fig.  53). 
After  circulating  through  the  lung,  and  undergoing  certain 
changes  there  which  will  subsequently  be  described  (see  p.  173), 
the  blood  is  collected  into  the  great  pulmonary  veins  which 
carry  it  back  to  the  base  of  the  heart,  and  discharge  it  into  the 
left  auricle.  By  a  similar  mechanical  arrangement  to  that  on 
the  right  side,  the  blood  passes  from  the  left  auricle  to  the  left 
ventricle,  and  from  the  left  ventricle  to  the  aorta. 

The  aorta  is  the  great  arterial  trunk  which  carries  the  blood 
from  the  heart  to  distribute  it  by  means  of  its  branches  to  the 
entire  system.  This  great  vessel  leaves  the  heart  near  the 
middle  of  the  base,  and  at  first  passes  (see  Fig.  54)  upwards 
and  to  the  right  (the  ascending  aorta) ^  but  soon  takes  a  sharp 
bend  to  the  left  and  turns  downward  (the  arch  of  the  aorta). 
In  its  descending  course  the  aorta  {descending  aorta)  lies  close 
to  the  vertebral  column  and  a  little  to  the  left  of  it.  From 
the  convexity  of  the  arch  of  the  aorta  there  arise  the  large 
arteries  which  carry  the  blood  to  the  head,  neck,  and  arms 
(see  Fig.  54),  viz.  the  innonmiate  artery  (dividing  into  the  right 
subclavian  and  right  common  carotid)  for  the  supply  of  the  right 
side,  and  the  left  common  carotid  and  left  subclavian  arteries 
for  the  left  side.  From  the  descending  thoracic  aorta  various 
branches  are  given  off,  the  chief  of  which  are  those  to  the 
tissue  of  the  lungs  called  the  bronchial  arteries,  and  the  inter- 
costal arteries  which  pass  outwards  on  each  side  under  each  pair 
of  ribs  to  supply  the  intercostal  muscles.  The  aorta  leaves  the 
thorax  at  the  back  of  the  diaphragm  (see  Fig.  52),  and  passes 
down  the  abdomen  as  the  abdominal  aorta,  lying  immediately  in 


Fig.  54. — General  view  of  the  heart  and  great  blood-vessels  of  the  trunk. 

A,  right  auricle  ;  £,  left  auricle  ;  C,  right  ventricle  ;  D,  margin  of  left  ventricle  ;  E,  ribs  ; 
J^,  kidneys  ;  i,  arch  of  the  aorta  ;  2,  descending  aorta  ;  3  and  4,  right  and  left  carotid 
arteries  ;  5  and  6,  right  and  left  subclavian  arteries  ;  7,  arteries  supplying  the  lower 
extremities;  8,  pulmonary  artery  ;  9,  vena  cava  superior;  10  and  ir,  right  and  left 
subclavian  veins ;  12  and  13,  right  and  left  jugular  veins  ;  14,  vtna  ca\a  inferior ; 
15  and  16,  veins  which  collect  blood  from  the  lower  extremities. 


84  Elementary  Physiology. 

front  of  the  vertebral  column.  Here  it  gives  off  large  branches 
to  supply  blood  to  the  stomach,  spleen,  liver,  intestines,  kidneys, 
and  other  abdominal  viscera,  and  finally  bifurcates  into  two 
large  arteries  called  the  common  iliacs,  each  of  which  later 
divides  into  two  branches,  viz.  the  internal  iliac^  for  the  supply 
of  the  viscera  in  the  lower  part  of  the  abdomen  {the pelvis);  and 
the  external  iliac^  which  furnishes  the  chief  blood-supply  to  the 
leg. 

The  heart,  and  the  roots  of  the  great  blood-vessels  arising 
from  it,  are  surrounded  by  a  strong  fibrous  bag  called  the 
pericardium^  which  is  attached  below  to  the  diaphragm  and 
above  to  the  great  vessels.  The  inner  surface  of  this  bag  is 
lined  by  a  smooth  membrane  {serous  membrane).  This  serous 
membrane  is  reflected  above  over  the  roots  of  the  great  vessels, 
and  is  continued  all  over  the  outer  surface  of  the  heart.  There 
are  thus  two  smooth  surfaces  opposed  to  each  other,  the  inner 
lining  of  the  pericardium  and  the  outer  lining  of  the  heart,  and 
between  these  surfaces  friction  is  reduced  to  a  minimum,  as 
the  heart  beats  within  the  pericardium.  The  amount  of 
friction  is  still  further  reduced  by  a  fluid  {pericardial  fluid) ^ 
which  is  secreted  by  this  inner  serous  coat  of  the  pericardium 
and  moistens  the  surface.  Besides  this  function  of  protecting 
the  heart  from  friction  as  it  beats,  the  pericardium  is  a  strong 
envelope  capable  of  protecting  the  thin-walled  auricles  from 
over  distension  and  injury  by  any  transient  engorgement  of  the 
heart  with  blood. 

The  mouth,  or  hiccal  cavity.,  opens  posteriorly  into  a  funnel- 
shaped  tube  (the  pharynx)  lined  by  muscular  walls,  and  this 
tube  narrows  as  it  descends  in  the  neck  into  the  oesophagus.,  or 
gullet  (see  Fig.  91,  p.  182).  The  oesophagus  is  also  a  tube  with 
muscular  walls  which  is  collapsed  unless  when  food  is  passing 
along  it  (see  p.  182),  and  in  the  neck  lies  behind  the  trachea,  or 
windpipe.  Both  these  tubes  enter  the  thorax  at  its  apex,  but 
while  the  trachea  soon  bifurcates  into  two  branches  called  the 
bronchi,  one  of  which  passes  to  each  lung,  the  oesophagus 
passes  straight  through  the  thorax,  pierces  the  diaphragm 
immediately  in  front  of  the  aorta,  and  widens  out  into  the 
stomach  soon  after  entering  the  abdomen. 


Position  of  tJie  Viscera.  85 

The  thymus  is  a  small  gland  resembling  in  its  structure  a 
lymphatic  gland,  which  lies  at  the  apex  of  the  thoracic  cavity, 
surrounding  the  trachea  and  in  front  of  it.  It  is  scarcely 
recognizable  as  a  gland  after  middle  life. 

Practically  the  whole  of  the  remaining  volume  of  the 
thoracic  cavity  is  occupied  by  the  two  lungs,  and  this  fraction 
of  the  space  greatly  exceeds  the  rest  The  lungs  are  moulded 
to  the  shape  of  the  thoracic  cavity.  The  diaphragm  beneath, 
forming  the  floor  of  the  thoracic  cavity,  is  convex  towards  the 
thorax,  and  the  bases  of  the  lungs  are  correspondingly  concave. 
The  diaphragm  is  placed  somewhat  lower  on  the  left  side,  and 
the  left  lung  is  in  consequence  longer  than  the  right,  but  it  is 
narrower,  and  more  room  is  notched  out  of  it  than  out  of  the 
right  to  accommodate  the  heart,  so  that  its  volume  with  an  equal 
amount  of  distension  is  slightly  less  than  that  of  the  right  lung. 
The  outer  surface  of  each  lung  is  convex,  to  fit  the  concavity  of 
the  thoracic  wall.  These  outer  surfaces  show  in  each  case  a 
deep  cleft  or  fissure,  which  begins  near  the  apex  at  the  posterior 
border  and  passes  obliquely  forward  and  downward  to  end 
near  the  front  of  the  lower  margin,  thus  dividing  each  lung 
into  two  lobes.  The  right  lung,  further,  has  a  cleft  running 
horizontally  forward  from  the  middle  of  the  oblique  one,  so 
that  it  is  divided  into  three  lobes.  The  inner  or  apposed 
surfaces  of  the  two  lungs  lie  against  each  other  except  where 
they  are  separated  by  the  heart,  great  vessels,  and  other 
structures  mentioned  above  ;  each  inner  surface  is  indented  to 
accommodate  these  structures.  The  apices  of  the  lungs  are 
somewhat  dome-shaped,  and  extend  up  to  the  neck  on  each 
side,  completely  fiUing  up  the  apex  of  the  thorax.  Each  lung 
in  a  healthy  condition  is  unattached  save  at  the  root  on  the 
inner  surface,  where  the  bronchus,  pulmonary  arteries  and 
veins,  and  bronchial  arteries  and  veins,^  as  well  as  certain 
nerves,  enter  and  leave  it. 

Each  lung  is  further  enclosed  in  a  double-walled  bag   or 

'  The  lung  tissue  cannot  be  nourished  by  the  venous  blood  commg  to 
it  from  the  heart,  but  must,  like  the  other  tissues  of  the  body,  be  supplied 
with  arterial  blood  ;  this  is  conveyed  by  special  arterial  branches  given  off 
from  the  aorta,  which  are  much  smaller  than  the  pulmonary  vessels,  and 
are  called  the  bronchial  arteries. 


S6  Elementary  Physiology. 

sac,  called  the  pleura.  One  layer  of  this  pleura  is  closely 
applied  to  the  inner  wall  of  the  thorax,  and  at  the  root  of  the 
lung  this  layer  is  reflected  over  the  outer  surface  of  the  lung, 
forming  a  second  layer  which  is  applied  to  the  lung  and  closely 
invests  it.  These  layers  of  the  pleura  are  exceedingly  thin, 
and  during  life  there  is  practically  no  space  between  them, 
the  lungs  distending  with  each  inspiration  and  filling  the  whole 
space.  But  if  an  opening  of  any  considerable  size  be  made  in 
the  wall  of  the  thorax,  the  outer  layer  adhering  closely  to  the 
thoracic  wall  is  cut  through  in  the  process,  the  elasticity  of  the 
distended  lung  comes  into  play,  the  lung  shrinks  and  air  is 
drawn  in  between  the  folds  of  the  pleura.  The  rhythmical 
enlargement  of  the  thorax  by  the  action  of  the  respiratory 
muscles  is  now  incompetent  to  draw  air  into  the  lungs. 
Instead  of  the  air  entering  and  leaving  the  lungs,  it  merely 
passes  in  and  out  through  the  artificial  opening  in  the  thoracic 
wall,  and  the  animal  soon  suffocates  and  dies  unless  some 
means  be  taken  to  blow  air  in  and  out  of  the  lungs.  Although 
the  lungs  throughout  life,  being  always  less  or  more  distended, 
have  ever  this  tendency  to  shrink  in  volume  and  leave  the 
thoracic  wall,  they  cannot  do  so,  because  a  vacuum  would  be 
thereby  formed^  The  thorax  is  an  air-tight  cavity,  and  as 
its  volume  is  rhythmically  altered  in  respiration,  air  must 
alternately  pass  into  and  out  of  the  lungs.  Hence,  in  a  normal 
condition  of  the  animal,  during  life  there  is  a  potential  space 
only  between  the  two  layers  of  each  pleura.  The  pleura  is  hence 
a  thin  double-walled  closed  sac,  both  layers  of  which  lie  in 
contact  with  each  other  surrounding  the  lung  and  continuous 
round  its  root.  A  very  small  amount  of  fluid  {pleiiritic  fltdd)  is 
normally  secreted  between  the  two  layers  of  the  pleura  and 
acts  as  a  lubricant ;  but  in  diseased  conditions  of  the  pleura 
(such  as  pleurisy)  the  amount  of  this  fluid  may  become 
enormously  increased,  and  the  two  layers  of  the  pleura  be 
widely  separated  by  it.  "  Since  the  volume  of  the  thorax  is 
limited,  any  large  accumulation  of  fluid  in  such  a  position 
becomes  dangerous,  because  the  lungs  collapse  to  a  corre- 
sponding extent.  Finally,  the  animal  dies,  if  the  condition  be 
not  relieved,  by  siffocation  or  asphyxia  ;  for  it  is  evident  that  a 


Position  of  the   Viscera.  Sy 

point  can  be  reached  when  it  is  impossible  to  suck  air  into  the 
hmgs  by  distension  of  the  thorax  in  presence  of  such  an 
accumulation  of  fluid  between  the  layers  of  the  pleura. 

The  Abdominal  Viscera. 

The  viscera  contained  in  the  abdominal  cavity  include 
the  lower  and  by  far  the  larger  portion  of  the  alimentary 
canal,  the  liver,  the  spleen,  the  pancreas,  the  suprarenal 
bodies,  the  kidneys,  and  the  bladder.  In  addition  to  these 
there  are  several  large  lymphatic  glands ;  certain  folds  of 
connective  tissue  which  suspend  or  attach  the  viscera  in  their 
various  positions,  and  carry  the  blood-vessels  which  supply 
them  with  blood ;  the  abdominal  aorta  and  inferior  vena 
cava,  passing  along  parallel  and  close  to  the  vertebral  column 
behind ;  certain  nerve  centres  or  ganglia  and  nerves  passing  to 
and  from  them  for  the  innervation  of  the  viscera ;  and,  in 
the  female  abdomen,  the  internal  generative  organs,  consisting 
of  the  uterus  or  womb,  with  its  appendages,  and  the  ovaries. 

These  soft  viscera  are  not  so  thoroughly  protected  by  a  complete  bony 
frame\York  as  are  those  of  the  thoracic  cavity,  nor  is  this  necessary  in  the 
case  of  the  abdomen.  Strong  broad  sheets  of  muscle  passing  from  the 
margin  of  the  bony  pelvis  below  to  the  lower  ribs  above  form  a  sufficient 
protection.  There  is  no  need  to  rhythmically  change  the  capacity  of  the 
abdominal  capacity  as  there  is  in  the  case  of  the  thorax,  for  there  is  here  no 
respiratory  organ,  like  a  lung,  into  and  out  of  which  air  must  alternately 
pass.  Further,  a  complete  bony  framework  surrounding  the  abdomen 
would  impede  the  action  of  the  diaphragm  by  fixing  the  position  of  the 
abdominal  viscera.  When  the  diaphragm  contracts,  it  forces  down  the 
contents  of  the  abdomen,  thus  enlarging  the  volume  of  the  thorax  and 
sucking  air  into  the  lungs  ;  the  abdominal  muscles  in  front  relax,  and 
allow  room  for  the  abdominal  viscera  by  an  increase  in  the  dimensions  of 
the  abdomen  from  front  to  back.  Thus  the  sheets  of  abdominal  muscles 
furnish  just  the  needed  kind  of  support  to  the  abdominal  viscera,  yielding, 
yet  firm,  and  suited  to  the  other  requirements  of  the  case.  Although  the 
volume  of  the  abdominal  cavity  does  not  vary  with  a  quick  rhythm,  like 
that  of  the  thorax,  yet  it  does  vary  very  considerably  from  time  to  time, 
being  distended  somewhat  after  each  meal,  on  account  of  the  increased 
volume  of  stomach  and  intestine.  Here,  again,  the  muscular  walls  are 
suited  admirably  for  plapng  their  required  part ;  by  relaxing  they  can  yield 
the  increased  volume,  and  by  an  increased  tonicity  they  can  become  more  con- 
tracted, and  give  a  uniform  degree  of  support  when  the  volume  diminishes. 


88 


Elementary  Physiology. 


The  inner  surface  of  the  abdomen,   including  the  under 
surface  of  the  diaphragm,  is  Uned  by  a  thin  smooth  dehcate 


Fig.  55. — The  viscera  of  the  thorax  and  abdomen,  viewed  from  the  front. 
X,    ribs,  the  front  portions  of  which,  together  with  the  sternum,  have  been  removed  ; 
2,  bones  of  the  pelvis  ;  3,  diaphragm  ;  4,  thorax  ;  5,  abdomen  ;  6,  right  lung  ;  7,  left 
lung  ;  8,  heart;  9,  stomach;   10,    right  lobe  of  the  liver  ;  11,  left  lobe  of  the  liver; 
12,  spleen;  13,  pancreas;   14,  small  intestine;  15,  large  intestine;  16,  bladder. 


Position  of  the   Viscera.  CS9 

serous  membrane  called  the  peritoneum.  This  smooth  mem- 
brane is  further  reflected  over  both  surfaces  of  the  stronger 
membranes  which  suspend  the  organs  (when  these  do  not  lie 
on  the  wall  of  the  cavity),  and  over  the  organs  themselves,  thus 
forming  a  very  complete  smooth  investment  for  all  the  organs. 
A  small  amount  of  fluid  is  secreted,  which  wets  the  surface  of 
the  peritoneum,  and  has  that  same  function  as  a  lubricant 
which  has  been  pointed  out  in  the  case  of  several  other  similar 
secretions. 

On  removing  the  abdominal  wall  in  front,  the  lower  part  of 
the  liver  is  seen  above  and  to  the  right  side.  But  the  greater  part 
of  this  organ  lies  beneath  the  lower  ribs,  being  moulded  to  the 
concavity  of  the  diaphragm,  against  which  its  upper  dome- 
shaped  surface  fits,  and  from  which,  and  the  abdominal  wall 
behind,  it  is  suspended  by  strong  folds  of  connective  tissue 
coated  with  peritoneum,  termed  ligaments.  It  is  divided  by  a 
fissure  into  a  right  and  left  lobe,  of  which  the  right  is  the 
larger.  On  the  right  side  additional  fissures  further  separate 
the  right  lobe  into  smaller  lobes,  called  the  quadrate.^  Spigelian, 
and  candate  lobes.  The  gall  bladder,  which  temporarily  receives 
the  bile  secreted  by  the  liver  before  it  is  poured  out  into  the 
intestine,  is  situated  in  a  depression  on  the  under  surface  of 
the  right  lobe  (see  Fig.  81,  p.  158). 

The  stomach  lies  lower  down  and  to  the  left  of  the  liver.  Its 
exact  position  and  the  extent  to  which  it  is  visible  on  opening 
the  abdominal  cavity,  depend  upon  the  amount  to  which  it  is 
distended  by  food,  but  the  greater  part  of  it  is  usually  con- 
cealed by  the  ribs  and  the  liver.  Its  upper  surface  touches  the 
diaphragm,  and  it  is  to  some  extent  suspended  from  this 
structure  by  the  oesophagus.  It  is  further  held  in  position 
by  ligamentous  bands  called  omenta^  which  connect  it  with 
the  liver,  intestine,  and  spleen.  The  stomach  is  an  enlarged 
portion  of  the  alimentary  canal,  which  holds  the  food  for  a 
time  after  a  meal,  until  it  can  gradually  be  passed  into  and 
along  the  lower  part  of  that  canal,  called  the  intestine,  or 
boiuel.  During  this  stay  in  the  stomach  part  of  the  food  is 
acted  on  chemically  by  a  fluid  secreted  by  certain  glands 
imbedded  in  the  inner  layer  of  its  walls  (see  p.  146).     After  a 


90  Elementary  Physiology. 

time  the  food  is  forced  on,  out  of  the  stomach  and  into  the 
intestine,  by  the  contraction  of  muscular  fibres  which  lie  in  the 
stomach  walls  external  to  this  glandular  layer.  The  junction 
of  oesophagus  with  stomach,  called  the  cardia,  is  kept  closed 
unless  when  food  is  passing  in,  by  a  ring  of  muscle  called  a 
sphincte7'.  A  similar  sphincter  guards  the  passage  from  stomach 
to  intestine  {pylorus)^  and  the  food  does  not  pass  continually 
from  stomach  to  intestine,  but  only  at  intervals,  when  the 
pyloric  sphincter  is  relaxed  and  the  muscular  coats  of  the 
stomach  contract  on  the  contained  food. 

A  double  fold  of  connective  tissue  coated  with  peritoneum,  called  the 
great  oment7im,  hangs  down  like  an  apron  from  the  lower  surface  of  the 
stomach,  and  usually  conceals  the  greater  part  of  the  intestine.  The  front 
fold  of  this  loop  turns  backwards  underneath,  and  is  continued  as  a  back 
fold,  which  passes  up  and  is  attached  to  the  transverse  colon  [vide  infra). 
On  removal  of  the  great  omentum,  the  arrangement  of  the  parts  of  the 
intestine  can  be  more  clearly  seen. 

The  intestine,  by  differences  in  structure  and  arrangement, 
is  divided  into  two  distinct  parts,  the  small  and  the  large 
intestine;  but  for  descriptive  purposes  the  small  intestine  is 
further  divided  into  three  parts,  termed  the  duodenum,  jepmum, 
and  ileum  respectively,  which  are  structurally  much  alike ;  and 
the  large  intestine  is  similarly  divided  into  ascending  colon, 
transverse  colon,  descending  colon,  sigmoid  flexure,  and  rectum. 

The  small  intestine  is  much  the  longer  portion  of  the 
alimentary  canal,  and  measures  in  man,  on  the  average,  about 
twenty  feet.  The  first  ten  to  twelve  inches  of  its  length  form 
a  (J-sbaped  loop  with  the  stomach,  in  the  bend  of  which  an 
important  gland  called  the  pancreas  lies.  This  portion  is 
called  the  duodenu,m.  The  upper  two-fifths  of  the  small  intes- 
tine is  called  the  jejunum,  and  the  lower  three-fifths  ileum ; 
but  there  is  no  demarcation  between  them,  and  the  names  are 
merely  used  to  specify  different  parts  of  the  length  of  the  small 
intestine. 

The  small  intestine  is  attached  to  the  abdominal  wall  at 
the  back  by  a  strong  sheet  of  ligament,  covered  on  both 
its  surfaces  by  peritoneal  membrane,  and  called  the  mesentery. 
This  mesentery  is  a  strong,  but  thin  and  transparent,  membrane, 


Position  of  the    Viscera.  91 

which  is  attached  by  one  border  to  the  abdominal  wall,  down 
the  mid  line  behind,  and  broadens  out  in  a  fan-shaped  fashion 
to  be  attached  to  the  whole  length  of  the  small  intestine  along 
its  opposite  border  in  front.  It  carries,  between  its  folds,  the 
vessels  conveying  the  blood  to  and  from  the  intestine,  as  well 
as  the  lymphatic  vessels  which  arise  in  the  walls  of  the  intes- 
tine by  minute  lymphatic  capillaries,  and  unite  to  form  large 
lymphatic  vessels  in  the  mesentery.^  The  arteries  run  out  like 
rays  in  a  divergent  fashion  along  the  mesentery  towards  the 
intestine,  and  on  nearing  it  join  together  and  form  arches, 
from  which  smaller  branches  pass  inwards  to  feed  the  intes- 
tinal wall  with  blood.  The  returning  veins  follow  the  course 
of  the  arteries,  and  unite  to  form  large  veins  which  lie  alongside 
the  arteries.  At  its  lower  end  the  small  intestine  opens  into 
the  large  intestine  by  an  orifice  which  is  closed  by  a  valve 
{ileo-ccecal  valve).,  formed  by  two  folds  arising  from  the  inner  part 
of  the  wall.  This  valve  permits  the  intestinal  contents  to  pass 
readily  from  the  small  to  the  large  intestine,  but  bars  all 
motion  in  the  opposite  direction.  The  entrance  of  the  small 
into  the  large  intestine  is  not  placed  quite  at  the  beginning  of 
the  large  intestine,  but  at  one  side  near  the  beginning.  That 
is  to  say,  the  small  and  large  intestine  are  not  joined  up  end  to 
end,  but  the  small  opens  into  the  large  intestine  laterally  at  a 
small  distance  from  the  end  of  the  large  intestine.  The  large 
intestine  has  hence  a  blind  end  or  sac,  which  is  termed  the 
c(Bcic7n.^  and  this  is  further  prolonged  into  a  small  finger-like 
projection  of  much  smaller  bore,  called  the  vermiform  appendix. 
The  length  and  capacity  of  the  caecum  vary  greatly  in  different 
animals ;  in  the  herbivora  (for  example,  in  the  rabbit)  it  is 
very  capacious,  while  in  the  carnivora  (such  as  the  cat  and  dog) 
it  is  rudimentary.     It  is  also  very  small  in  man. 

The  small  intestine  lies  in  coils  in  the  abdominal  cavity 
below  the  liver  and  stomach.  It  occupies  the  greater  part  of 
the  space,  and  is  surrounded  by  the  large  intestine,  which  it 
joins  in  the  lower  part  of  the  abdomen  on  the  right  side  (see 

^  These  mesenteric  lymphatics  are  often  termed  lacteals,  because  they 
contain  a  milky  white  emulsion  of  fat  when  fat  absorption  is  going  on 
(seep.  156). 


92  Elementary  Physiology. 

Fig.  55).  The  large  intestifte,  or  coloji,  has  in  man  an  average 
length  of  five  to  six  feet.  It  passes  upwards  on  the  right  side 
of  the  abdomen  as  the  ascendmg  colon.  It  then  arches  over  to 
the  left  in  a  horizontal  portion  known  as  the  transverse  colo7i, 
which  is  seen  in  front  (see  Fig.  55)  after  removal  of  the  great 
omentum,  lying  between  the  stomach  and  the  folds  of  the  small 
intestine.  It  next  bends  backward  and  downward,  and  passes 
down  posteriorly  to  the  small  intestine  as  the  descending  colon 
on  the  left  side  of  the  abdominal  cavity.  The  descending 
colon  bends  towards  the  middle  line  in  the  lower  and  posterior 
part  of  ths  abdominal  cavity,  and  this  bent  portion  is  termed 
the  sig7noid  flexitre.  Finally,  the  sigmoid  flexure  leads  to  a 
straight  tube  with  strongly  developed  muscular  walls  called  the 
rectum,  which  ends  in  the  external  opening  of  the  intestine 
known  as  the  amis. 

The  large  intestine  is  fixed  in  position  by  folds  of 
peritoneum  which  surround  it.  The  transverse  colon  is  less 
closely  fixed  in  this  fashion  than  the  other  parts,  being  loosely 
attached  to  the  abdominal  wall  at  the  back  by  a  long  fold  of 
peritoneum  {the  transverse  vieso-colon).  On  the  other  hand,  the 
peritoneum  does  not  usually  completely  encircle  the  ascending 
or  descending  colon,  but  passes  over  them  in  a  single  sheet, 
which  touches  the  intestine  only  for  about  two-thirds  of  its 
circumference,  and  is  then  reflected  on  to  the  abdominal  wall, 
so  firmly  fixing  the  intestine  in  position. 

The  spleen  cannot  be  seen  from  the  front  unless  by  dis- 
placing the  stomach,  as  it  is  deeply  placed  behind  and  to  the 
leftside  of  that  organ  in  the  left  iLpper  part  of  the  abdominal  cavity. 

The  spleen  is  an  elongated  dark  red  coloured  body  filled  with  blood, 
which  enters  it  along  one  side  (which  is  concave  and  lies  against  the 
stomach)  by  a  number  of  fairly  large-sized  arteries.  These  arteries  sub- 
divide within  the  substance  of  the  spleen  and  finally  give  rise  to  a  mesh- 
work  of  capillaries  which  open  out  into  spaces  or  sinuses  in  the  tissue  of 
the  organ  without  any  definite  lining  or  wall.  The  blood  is  collected  up 
again  from  these  sinuses  by  capillaries  leading  to  small  veins  which  unite  to 
form  the  splenic  veins,  and  these  issue  from  the  spleen  alongside  the 
arteries.  The  function  of  the  spleen  is  not  clearly  known.  It  is  not 
essential  to  life,  for  animals  continue  to  live  in  good  health  after  it  has  been 
completely  removed.  It  has  the  property  of  rhythmically  contracting  at 
intervals  of  about  once  a  minute.     These  contractions  are  brought  about 


Position  of  tJie   Viscera.  93 

by  strands  of  involuntary  muscle  fibre  which  pass  inward  along  septa  of 
connective  tissue  called  trabeada:.  The  trabeculoe  arise  from  the  outer 
sheath  of  the  organ  and  pass  inward,  forming  a  framework  from  which 
lesser  strands  of  connective  tissue  arise  so  as  to  make  a  network.  The 
cells  of  the  spleen  are  contained  in  this  network  as  well  as  the  capillaries 
and  sinuses  above  mentioned.  It  is  supposed  that  the  spleen  possesses  the 
power  of  destroying  the  effete  or  ivorn-oiit  red  corpuscles  of  the  Mood  (see 
p.  121),  and  some  observers  claim  to  have  observed  microscopically  such  a 
process  of  destruction  going  on  in  the  spleen  cells  ;  still,  the  evidence  on 
this  point  is  not  very  convincing.  The  anterior  surface  of  the  spleen  at 
which  the  blood-vessels  enter  and  leave  is  concave,  and  attached  to  the 
stonidich.  by  the  gas tro-sJ)/emc  omeutum,  a  strong  ligamentous  band,  coated 
over  like  everything  else  in  the  abdominal  cavity  by  a  reflexion  of  the 
peritoneal  lining.  The  outer  or  posterior  surface  of  the  spleen  is  convex 
and  unattached,  it  touches  the  diaphragm  by  its  upper  border,  and  lies 
obliquely  against  the  posterior  abdominal  wall.  The  upper  end  is  placed 
close  to  the  left  suprarenal  body  {vide  infra)  near  the  spine,  and  from  this 
point  the  organ  lies  outward  and  downwards,  following  the  course  of  the 
diaphragm  for  about  halfway  round  the  side. 

The  kidneys  are  two  oval  or  bean-shaped  bodies  of  a  deep 
red  colour,  each  about  four  inches  long,  two  and  a  half  inches 
in  breadth,  and  rather  more  than  an  inch  thick,  which  lie  at 
the  back  of  the  abdomen  on  each  side  of  the  mid-line,  opposite 
the  last  dorsal  and  upper  two  or  three  lumbar  vertebra.  Their 
position  is  slightly  oblique,  the  upper  end  being  nearer  to  the 
spine  than  the  lower,  and  the  left  is  placed  slightly  higher  than 
the  right  kidney.  The  external  position  opposite  to  which  the 
kidneys  lie  is  immediately  beneath  the  last  rib  at  the  back, 
just  on  each  side  of  the  vertebral  column.  The  kidneys  are 
embedded  in  a  mass  of  fat,  and  lie  behind  the  peritoneal  lining 
of  the  abdominal  cavity,  which  is  reflected  over  them  in  front 
as  they  bulge  out  into  the  abdominal  cavity.  The  blood- 
vessels enter  at  the  inner  border  where  there  is  a  depression 
in  the  surface  called  the  hilnni  ;  here  also  the  duct,  or  ureter^ 
which  conveys  away  the  excreted  urine,  leaves  the  kidney. 
The  ureters  are  two  long  slender  tubes,  one  on  each  side, 
which  leave  the  kidneys  and  pass  to  the  bladder,  into  which 
they  continuously  pour  the  urine  as  it  is  secreted  by  the 
kidneys.  They  pass  through  the  bladder  wall  very  obliquely, 
and  this  produces  a  valve-like  effect.  For  when  the  pressure 
is  from  the  ureter  to  the  bladder,  the  ureters  remain  open ;  but 


94  Elementary  Physiology. 

when  the  pressure  inside  the  bladder  is  increased,  as  is  the 
case  when  it  is  being  emptied,  the  increased  pressure  forces 
against  each  other  the  layers  of  the  bladder  wall,  and  so  the 
mouths  of  the  ureters,  where  these  pass  through  obliquely,  are 
closed,  and  no  passage  of  urine  from  the  bladder  up  the  ureters 
can  take  place. 

The  urinary  bladder  is  a  distensible  bag  which  serves  to 
collect  the  urine  between  the  periods  of  its  discharge  from  the 
body.  It  lies  in  the  mid-line  at  the  lower  or  pelvic  part  of 
the  abdomen,  beneath  the  arch  formed  by  the  pubic  bones,  but 
may  when  distended  appear  above  them.  When  distended  it 
has  a  rounded  or  egg-shaped  form,  the  broader  end  being 
towards  the  base,  or  fundus,  which  rests  against  the  rectum 
behind.  There  are  three  openings  to  the  bladder,  of  which 
two,  those  of  the  ureters,  have  already  been  described ;  these 
enter  posteriorly  at  the  upper  part  of  the  base  on  each  side. 
The  other  opening  is  the  urethra^  by  which  the  urine  leaves  the 
bladder. 

The  urethra  is  a  tube  with  muscular  walls,  which  leaves  the 
bladder  at  its  lower  part  or  neck.  It  is  kept  closed,  except 
when  urine  is  being  discharged  from  the  bladder  by  a  muscular 
ring  or  sphincter  placed  where  it  leaves  the  bladder. 

The  suprarenal  bodies  are  two  small  yellow  coloured  glands 
situated  one  immediately  above  each  kidney  and  close  to  the 
middle  line.  These  bodies,  though  inconspicuous  in  size,  are 
of  vital  importance  in  the  body,  for  it  is  impossible  to  remove 
them  without  causing  death.  They  are  glandular  in  structure, 
and  although  they  possess  no  ducts,  it  is  probable  that  the 
cells  secrete  material  which  is  poured  directly  into  the  blood- 
stream. A  diseased  condition  of  these  bodies  is  associated 
with  a  peculiar  and  fatal  malady  (Addison's  disease),  which 
is,  however,  comparatively  rare.  A  characteristic  sign  of  this 
disease  is  a  remarkable  bronzing  of  the  skin  in  patches ;  this  is 
accompanied  by  muscular  weakness,  and  a  fall  of  the  pressure 
of  the  blood  in  the  arteries.  It  has  recently  been  discovered 
that  extracts  of  these  glands  in  water,  when  injected  into  the 
veins  of  other  animals,  possess  the  property  of  greatly  raising 
the  pressure  of  the  blood  in  the  arteries,  by  constricting  the 


Position  of  the  Viscera.  95 

small  arterioles  all  over  the  body,  and  hence  preventing  so 
rapid  an  escape  of  the  blood  through  these,  as  it  is  pumped 
into  the  arteries  from  the  heart.  It  has  hence  been  suggested 
that  the  suprarenal  bodies  furnish  to  the  blood  an  iiiter7ial 
secretion  which  is  essential  for  the  maintenance  of  the  tonicity 
of  the  muscle  fibres  surrounding  the  arterioles. 

Another  important  ductless  gland  which  may  be  con- 
veniently mentioned  here,  although  it  is  not  contained  in  the 
abdomen,  is  the  thyroid  gland,  which  is  situated  in  the  neck,  on 
each  side  of  the  trachea,  near  the  thyroid  cartilage.  Complete 
removal  of  this  gland  in  man,  in  cases  where  it  has  been 
diseased,  has  been  found  to  cause  death,  and  a  similar  result 
has  been  obtained  in  the  case  of  certain  classes  of  animals. 
The  symptoms  most  closely  resemble  those  seen  in  man,  if 
monkeys  are  used  for  the  experiment.  In  other  animals,  such 
as  the  dog,  death  occurs  before  certain  of  the  symptoms  have 
time  to  become  manifest.  The  most  remarkable  symptom  is 
a  swollen  condition  of  the  connective  tissue  beneath  the  skin, 
producing  a  kind  of  artificial  cretinism,  which  is  known  as 
operative  myxoidema,  this  is  accompanied  by  symptoms  of  a 
disturbance  of  the  central  nervous  system  in  the  form  of 
tremors,  spasms,  and  convulsions.  These  disturbances  increase 
with  the  lapse  of  time,  and  finally  lead  to  death.  The  condition 
is  prevented  or  palliated  by  grafting  of  fresh  thyroid  tissue 
under  the  skin,  or  by  feeding  with  fresh  thyroid  glands  obtained 
from  other  animals.  Much  service  has  been  done,  by  the 
knowledge  thus  acquired,  in  practical  medicine  by  feeding 
patients  similarly  affected  on  the  fresh  glands  of  the  sheep 
and  other  animals. 

The  good  effects  obtained  by  grafting  the  thyroid  and  by 
thyroid  feeding,  show  that  the  thyroid  secretes  some  substance 
which  has  a  beneficial  action  in  the  body,  and  negative  the 
theory^  which  has  been  proposed  that  the  purpose  of  the 
thyroid  and  similar  glands  is  to  remove  from  the  blood  certain 
noxious  substances  which  tend  to  accumulate  therein. 

^  This  theory  is  termed  the  aiito-intoxication  theory,  in  contra- 
distinction to  the  other,  which  is  known  as  the  theory  of  internal 
secretion. 


CHAPTER  V. 

THE    CIRCULATORY  SYSTEM. 

The  circulation  of  the  blood,  carrying  nutriment  to  all  the 
tissues  of  the  body  and  removing  their  waste  products,  is 
maintained  by  means  of  the  heart  and  the  system  of  tubes 
connected  with  it  known  as  the  blood-vessels. 

Those  vessels  which  carry  the  blood  away  from  the  heart 
are  called  arteries.^  and  those  which  carry  it  back  to  the  heart 
are  called  veins.  Between  these  two  systems  of  vessels  there 
is  communication  in  the  tissues  by  an  immense  number  of 
minute  vessels,  called  capillaries.,  which  have  exceedingly  thin 
walls.  It  is  here,  in  the  capillaries,  that  the  real  work  of  the 
blood  is  done ;  through  the  delicate  walls  of  the  capillaries, 
free  interchange  of  nutrient  materials  and  waste  products  takes 
place ;  while  the  larger  vessels  (arteries  and  veins)  have  thick 
impermeable  walls,  and  merely  serve  the  purpose  of  conveying 
and  distributing  the  blood  to  the  capillaries.  The  entire 
blood-vascular  system,  consisting  of  heart,  arteries,  capillaries, 
and  veins,  is  completely  lined  internally  by  an  exceedingly 
thin  layer  of  flat  pavement  cells,  which  touch  each  other,  and 
dovetail  into  each  other  by  their  thin  edges.  These  flat  cells 
are  elongated  in  form,  and  each  possesses  a  nucleus  (see  Figs. 
56  and  59).  In  the  case  of  the  capillaries,  they  form  the  entire 
thickness  of  the  wall,  and  hence  there  is  free  diffusion  of  sub- 
stances in  solution  between  the  lymph  (bathing  the  tissue  and 
its  cells)  outside  the  capillary,  and  the  blood  inside  the 
capillary.  Also,  at  the  cell  junctions  there  are  minute  apertures 
through  which  the  white  cells  (leucocytes)  present  in  the  blood 
can  pass  from  the  capillary  into  the  lymph  spaces  without. 
The  capillaries  branch  and  anastomose  freely  with  one  another, 


TJie  Circulatory  System. 


97 


and  unite  at    their    ends    to  form    arterioles  and  venules,  the 
arterioles  being  next  the  arteries,  and  the  venules  next  the 


Fig    s6.-A  small  artery,  A,  and  vein,  J",  from  the  subcutaneous  connective  tissue  of  the 

rat.  treated  with  nitrate  of  silver.     (,175  diameters.; 
^   n   endothelial  cells  with  b,  b,  their  nuclei  ;  m,  m,  transverse  markings  due  to  staining 
'     orsubstance' between   the   muscular  fibre  cells ;    c,   c,  nuclei    of  connective-tissue 
corpuscles  attached  to  exterior  of  vessel. 

veins.  In  these  small  arteries  and  veins  there  are  disposed 
outside  the  epitheloid  coat  (endothelium)  layers  of  involuntary 
muscle  fibres,  which  are   arranged  in  a   circular  and  slightly 

H 


98 


Elementary  Physiology. 


spiral  fashion  around  the  tiny  vessel.  These  are  found  at  the 
places  nearer  the  capillary  as  isolated  cells  and  incomplete 
layers ;  but  as  the  distance  from  the  capillary  increases,  there 
is  first  formed  a  complete  layer  of  the  involuntary  muscle 
fibres,  and  at  points  still  more  remote  the  muscle  cells  are 
several  layers  thick. ^  At  the  same  time,  a  fine  layer  of  elastic 
fibres  forming  an  elastic  membrane  is  found  between  the  inner 
lining  of  pavement  cells  and  the  layers  of  muscle  fibres.  The 
muscular  coat  is  much  thicker  in  the  small  arteries  than  in  the 
small  veins,  and  the  elastic  coat  outside  the  endothelium  is 
also  much  more  strongly  developed.  As  the  arteries  and  veins 
increase   in   bore  their  walls   also  increase  in  thickness,  and 


Fig.  57. —Transverse  section  of  part  of  the  wall  of  the  posterior  tibial  artery. 

(75  diameters.) 

a,  epithelial  and  subepithelial  layers  of  inner  coat ;  b,  elastic  layer  (fenestrated  mem- 
brane) of  inner  coat,  appearing  as  a  bright  line  in  section  ;  c,  muscular  layer  (middle 
coat) ;  d,  outer  coat,  consisting  of  connective-tissue  bundles.  In  the  interstices  of 
the  bundles  are  some  connective-tissue  nuclei,  and,  especially  near  the  muscular  coat, 
a  number  of  elastic  fibres  cut  across. 

there  appears  in  addition  an  external  coat  {areolar  coat  or 
timica  adventitia)  composed  of  connective-tissue  fibres.  Many 
of  these  fibres  are  elastic ;  especially  in  the  arteries  which  require 
more  elastic  distensibility  than  the  veins.  It  is  this  external 
coat  which  gives  their  great  strength  to  the  large  vessels,  and 
especially  to  the  arteries  in  which  it  is  well  developed.  In  the 
very  largest  arteries,  such  as  the  aorta  and  its  pfimary  branches, 
the  middle  or  muscular  coat  and  the  outer  coat  become  to  a 
certain  extent  blended  so  that  there  is  an  admixture  of  elastic 
and  muscular  fibres. 

In  a  typical  medium-sized  artery  (see  Fig.  57)   the  wall  is 

^  The  purpose  of  this  muscular  coat  in  the  arterioles,  and  the  manner  in 
which  it  regulates  the  supply  of  blood  to  a  given  part,  have  already  been 
described  (see  pp.  57,  60). 


TJie  Ciradatory  System. 


99 


usually  described  as  consisting  of  three  coats,  namely,  an  imier 
or  elastic  coat  {tunica  intima),  consisting  of  the  innermost  pave- 
ment layer  or  endothelium  and  the  elastic  membrane  \  a  middle 


Fig.  58.— Transverse  section  of  part  of  the  wall  of  one  of  the  posterior  tibial 

veins  (man), 
a,   epithelial  and  subepithelial    layers   of  inner    coat ;    b,  elastic  layers  of  inner  coat : 
6-,  middle  coat  consisting  of  irregular  layers  of  muscular  tissue,  alternating  with  con- 
nective tissue,  and  passing  somewhat  gradually  into  the  outer  connective  tissue  and 
elastic  coat,  d. 

or  muscular  coat  (tunica  media),  consisting  of  concentric  non- 
striated  muscle  cells ;  and  an  external  or  areolar  coat  {tunica 
cxtima  or  tunica  adventitia).  The  larger  veins 
very  closely  resemble  the  arteries  in  structure, 
but  their  walls  are  much  thinner,  as  they  con- 
tain both  less  muscular  and  less  elastic  tissue 
(compare  Figs.  57,  58). 

On  account  of  the  large  amount  of  elastic 
tissue  which  the  arterial  walls  contain,  they  are 
capable  of  being  distended  when  the  pressure 
within  them  is  increased,  and  of  regaining 
their  original  bore  when  this  pressure  de- 
creases again.  The  veins,  on  the  other  hand, 
are  not  so  distensible  by  pressure.  The  united 
cross  section  of  the  large  veins  is  about  double 
that  of  the  correspondingly  large  arteries,  and  f 
they  are  never  quite  distended  by  blood  under    [  fj 


normal  conditions. 

The  heart  is  a  double  force-pump  contain-  fig.  59.  —  Epithelial 
ing  four  chambers,  two  of  which  are  placed  on      tSklr  tib"S'a?te°y' 
the  right  side,  and  together  constitute  what  is      ^'^^°  diameters.) 
often  termed  the  right  heart,  and  two  on  the  left  side,  forming 
the  left  heart.     This  duplicate  arrangement  is  necessary  because 


100 


Elementary  Physiology. 


Fig.  6o.' 


the  entire  course  of  the  circulation  forms  two  nearly  complete 
circuits,  as  shown  in   the    accompanying    diagram  (Fig.  6i), 

and  a  given  quantity  of  blood 
in  a  complete  round  comes 
twice  to  the  heart. 

The  upper  chamber  on 
each  side  is  thin  walled,  and 
incapable  of  exerting  or  with- 
standing any  great  amount  of 
pressure.  It  is  called  the 
auricle^  and  its  purpose  is,  by 
a  preliminary  contraction,^  to 
completely  fill  the  lower  cham- 
ber or  ventricle  before  this 
contracts  and  drives  the  blood 

'.°hyToYd"i^"e?;"t4oikme."rT"°'  '"^  ^^e   large  artery  leading 

away  from  it. 

The  ventricle  on  each  side  is  a  chamber  with  very  thick 
muscular  walls  capable  of  exerting  considerable  pressure,  when 
it  contracts,  upon  the  blood  contained  within  it.  There  is  a 
valve  arranged  between  each  ventricle  and  its  corresponding 
auricle  which  opens  towards  the  ventricle,  and  another  between 
each  ventricle  and  the  large  artery  which  issues  from  it,  open- 
ing towards  the  artery  (see  Figs.  65,  (^6).  On  account  of  these 
valves  the  blood  can  only  move  in  one  direction  when  the  ventri- 
cular wall  contracts  upon  it,  namely,  from  the  ventricle  into  the 
artery. 

There  is  no  valve  placed  between  each  auricle  and  the 
great  veins  which  enter  it,  in  order  to  prevent  the  blood  from 
flowing,  when  the  auricle  contracts,  from  the  auricle  into  these 
great  veins,  instead  of  into  the  ventricle,  because  there  is  less 
resistance  in  the  direction  of  the  ventricle  than  in  the  direction 
of  the  veins,  and  the  auricle  never  gets  up  sufficient  pressure 
under  normal  conditions  to  force  the  blood  back  into  the  veins 
instead  of  into  the  ventricle.  When  the  auricle  contracts,  the 
blood  is  not  under  pressure  in  the  ventricle,  and  the  effect  of 

^  The  contraction  of  either  auricle  or  ventricle  is  known  as  its  systole, 
and  its  relaxed  or  uncontracted  condition  as  its  diastole. 


The  Circulatory  System. 


lOI 


the   auricular   contraction    is    merely   to    slightly   distend   the 
ventricle  and  to  float  the  anriado-vcntricular  valve  ^  {i.e.  valve 


Fig.  6i. — Schematic  diagram  to  illustrate  the  course  of  the  circulation. 
I,   right  auricle;    2,   left  auricle;    3,   right   ventricle;    4,  left  ventricle;    5,  vena  cava 
superior  ;    6,   vena  cava  inferior  ;    7,  pulmonary'  arteries  ;  8,   lungs  ;  g,   pulmonary 
veins;  10,  aorta;  11,  vessels  of  alimentarj'  canal;  12,  vessels  of  liver;  13,  hepatic 
arterj-;   14,  portal  vein  ;  15,  hepatic  vein. 

^  The  auriculo -ventricular  valve  on  the  left  side  is  called  the  viitral 
valve,  that  on  the  right  the  tircnspid  valve.  The  valves  placed  on  the  aorta 
and  pulmonary  arteries,  as  they  respectively  issue  from  the  left  and  right 
ventricles,  are  called  the  aortic  and  pulmonary  seniilimar  valves. 


102  Elementary  Physiology. 

between  auricle  and  ventricle)  up  into  a  closed  position.  The 
main  work  at  each  heart-beat  thus  comes  on  the  thick-walled 
ventricles  which  have  to  force  the  blood  into  the  arteries,  where 
there  exists  a  considerable  pressure  {ai'terial  pressttre)^  the 
cause  and  need  of  which  will  be  presently  considered. 

Beginning  at  the  point  where  the  blood  is  poured  into  the 
right  auricle  by  the  superior  and  inferior  vence,  cavce,  the  course 
of  a  complete  circulation  may  be  indicated  as  follows  (see  Fig. 
6i).  The  blood  flows  into  the  right  auricle,  and  from  this, 
during  the  pause  before  a  heart-beat,  freely  into  the  right 
ventricle;  further,  an  additional  quantity  is  helped  in  before 
the  contraction  (systole)  of  the  right  ventricle  by  the  preced- 
ing contraction  (systole)  of  the  right  auricle.  From  the  right 
ventricle  the  blood  is  pumped  by  the  systole  into  the  pulmonary 
artery,  which  branches  and  carries  it  to  the  lungs.  Here  the 
blood  is  spread  out  in  a  thin  layer  by  a  vast  meshwork  of 
capillaries,  and  is  only  separated  from  the  air  filling  the  air 
spaces  {air  cells)  of  the  lung  by  the  thin  walls  of  these  capillaries. 
While  passing  through  these  capillaries  gaseous  interchange 
takes  place  between  blood  and  air,  in  consequence  of  which 
the  blood  loses  carbon  dioxide  formed  in  the  various  tissues 
by  oxidation  processes  going  on  there,  and  takes  up  a 
charge  of  oxygen  for  use  in  the  tissues  in  the  next  round. 
In  consequence  of  these  changes  in  the  gases  it  holds,  the 
blood  changes  in  colour  from  dark  purple  to  bright  scarlet,  or, 
as  it  is  termed,  is  changed  from  venous  to  arterial  blood.^ 
The  blood  is  gathered  up  from  the  lung  capillaries  by  veins, 
which  unite  with  one  another  to  form  larger  venous  trunks, 
and  leave  the  lungs  as  the  pulmonary  veins,  which  carry  the 
arterialized  blood  back  to  the  heart.  Here  it  enters  the  left 
auricle,  and  passes,  in  a  similar  manner  to  that  on  the  right 
side,  into  the  left  ventricle.  By  the  systole  of  the  left  auricle, 
the  left  ventricle  is  completely  filled;  then  this  chamber 
passes  into  systole,  and  the  blood  is  driven  under  pressure  into 
the  aorta.  The  aorta  and  its  various  branches  distribute  the 
blood  to  all  parts  of  the  body,  some  passes  to  this  organ  and 

^  The  nature  and  mode  of  these  changes  in  the  blood  will  be  more  fully 
discussed  elsewhere  (see  pp.  173,  ef  seq.). 


The  Circulatory  System,  103 

some  to  that,  and  after  circulating  through  the  capillaries  of 
the  part,  each  portion  is  collected  up  by  veins,  which  gradually 
unite  and  increase  in  size.  The  larger  veins  pour  their  contents 
into  the  great  venous  trunks,  the  superior  and  inferior  ve?ice 
cavcE,  and  these  empty  the  blood  into  the  right  auricle,  so  com- 
pletmg  the  circuit. 

The  circuit  between  right  auricle  and  left  ventricle  via  the 
lungs  is  termed  the  lesser  or  pulmonary  circulation,  and  that 
from  left  ventricle  back  to  right  auricle  is  known  as  the  greater 
or  systemic  circulation.  These  pumping  operations  by  the  two 
force-pumps  on  the  right  and  left  side  of  the  heart  go  on 
simulta?ieo2isly .  First,  both  auricles  contract  at  the  same 
instant  {auricular  systole),  then,  immediately  after,  both  ven- 
tricles contract  together  {I'eutricular  systole),  and  there  follows 
a  pause  {diastolic  pause),  during  which  all  four  chambers  are 
relaxed.  This  whole  cycle  of  events  occupies  about  y^  to  ^ 
of  a  second,  of  which  about  y^  of  a  second  is  taken  by  the 
auricular  systole,  y^  of  a  second  by  the  ventricular  systole,  and 
the  remainder  of  yo  to  3^  of  a  second  by  the  diastolic  pause.^ 

Certain  sounds,  known  as  the  heart  sounds,  are  heard  when 
the  ear  is  applied  to  the  chest  of  another  person  over  the  region 
of  the  heart,  or  may  be  heard  in  one's  self  when  lying  quietly  on 
one  side  in  bed.  Two  sounds  are  heard,  one  immediately  after 
the  other,  followed  by  a  pause.  The  first  sound  is  lower  in 
pitch  and  more  prolonged  than  the  second,  which  is  short  and 
sharp  like  that  produced  by  softly  plucking  a  piece  of  linen 
cloth,  such  as  a  pocket-handkerchief.  The  sounds  have  been 
compared  to  those  made  in  pronouncing  the  syllables,  lubb, 
dupp — lubb,  dupp. 

The  first  sound  is  produced  partially  by  the  contraction 
of  the  muscular  substance  of  the  ventricular  walls,  and  partially 
by  the  rush  of  the  blood  from  the  ventricles  into  the  arteries. 
The   second   sound  is   made   by   the    closure    of    the    valves 

^  The  rate  of  the  heart-beat  varies  under  different  conditions  from  time 
to  time,  being  increased  by  exercise,  excitement,  or  fever,  and  lessened  by 
repose,  assuming  the  recumbent  position,  or  sleeping.  It  also  varies  with 
age,  being  about  120  to  150  per  minute  in  the  new-born  infant,  80  to  90  in 
the  child,  70  to  80  in  the  adult,  and  50  to  60  in  old  age.  It  must  be 
understood,  however,  that  the  individual  variations  are  very  considerable. 


I04  Elementary  Physiology. 

guarding  the  entrances  of  the  aorta  and  pulmonary  arteries. 
For  the  pressure  in  the  ventricles  when  their  contraction  is 
over  falls  below  that  in  the  arteries,  and  these  valves  then 
fill  out  and  close  together  to  keep  the  blood  from  rushing 
back  from  the  arteries  into  the  ventricles  again.  In  conse- 
quence, the  first  heart  sound  occurs  during  the  ventricular 
systole,  and  the  second  at  the  commencement  of  the  diastolic 
pause,  just  as  the  ventricles  relax. 

These  sounds  are  of  great  importance  in  practical  medicine, 
because  they  become  altered  in  many  diseased  conditions  of 
the  heart,  and  the  character  of  the  alteration  gives  a  clue  to  the 
nature  of  the  disease.  The  physician  listens  to  them  by  means 
of  an  instrument  called  a  stethoscope,  which  is  in  principle  a 
hollow  tube,  or  double  tube,  serving  to  convey  the  sounds  from 
a  small  area  of  the  chest  wall  to  the  observers  ear. 

Since  the  right  and  left  ventricle  beat  at  exactly  the  same 
rate,  and  all  the  fluid  sent  from  the  right  ventricle  to  the 
lungs  must  be  afterwards  sent  from  the  left  ventricle  round 
the  system,^  it  follows  that  the  volume  of  blood  discharged 
at  each  systole  from  the  right  ventricle  must  be  equal  to  that 
discharged  from  the  left,  and  as  the  volume  of  the  ventricle 
must  become  adapted  to  the  volume  which  it  discharges  con- 
tinually during  life,  it  also  follows  that  the  internal  volume  of 
the  two  ventricles  must  be  the  same.  It  is  difficult  to  deter- 
mine this  volume  accurately,  but  it  has  been  estimated  at  4  to 
5  ounces,  or  100  to  120  cubic  centimetres.^ 

Although  the  entire  vascular  system  possesses  the  exceedingly 
smooth  epitheloid  lining  above  described,  which  has  the  effect 
of  diminishing  the  resistance  to  the  flow  of  the  blood  through 
it  to  a  minimum,  yet  there  is  a  considerable  resistance  to  the 
flow,  which   is   practically  all   due   to   the   fine   bore    of  the 

^  With  the  exception  of  a  comparatively  negligible  quantity  lost  by 
evaporation  in  the  kings. 

'^  That  is,  a  little  more  than  a  medium-sized  hen's  egg  ;  hence  when  the 
rate  at  which  the  heart  beats  is  taken  into  account,  the  vast  volume  of  blood 
sent  through  the  heart  per  day  is  realized.  To  obtain  the  day's  work  of 
the  heart  this  volume  must  be  multiplied  by  the  sum  of  the  pressures  at 
which  it  is  driven  into  the  aorta  and  pulmonary  artery ;  this  is  equal  to  a 
pressure  of  about  8  feet  of  water,  so  that  the  amount  of  work  done  daily 
by  the  heart  is  very  considerable. 


The  Circulatory  System.  105 

capillaries  and  the  smaller  arteries  and  veins  adjoining  them. 
This  form  of  resistance  is  what  is  known  as  fluid  resistance. 
When  a  fluid  flows  along  in  a  channel  or  tube,  there  is  a 
certain  amount  of  resistance  to  the  flow  due  to  the  roughness 
of  the  inner  wall  in  impeding  the  flow  of  the  layers  of  fluid 
nearest  to  it,  called  the  skin  resistance ;  this  is  reduced  to  a 
minimum  in  a  normal  condition  of  the  blood-vessels  of  the 
body  by  the  smoothness  of  their  internal  coat.  But,  besides 
this,  there  is  a  friction,  due  to  the  movement  of  the  layers  of 
fluid  on  each  other.  Those  portions  of  fluid  in  the  central 
part  of  the  bore  of  the  tube  move  more  rapidly  than  portions 
nearer  the  wall  of  the  tube,  thus  there  is  a  brushing  of  the 
fluid  against  itself,  which  impedes  and  delays  its  motion  as  a 
whole.  The  amount  of  this  resistance  varies  with  the  velocity 
of  the  fluid,  with  its  viscosity^  or  internal  friction,  and  with  the 
hore  of  the  tube.  As  the  bore  decreases,  the  resistance  becomes 
enormously  increased,  so  that  it  requires  a  great  pressure  to 
drive  fluid  through  very  fine  tubes  with  any  considerable 
velocity.  Now,  the  blood  is  a  fluid  with  considerable  viscosity, 
and  the  capillaries  through  which  it  has  to  be  driven  are 
exceedingly  narrow,  so  that  to  accomplish  the  purpose  the 
pressure  in  the  arteries  {arterial  blood-pressure)  must  be  main- 
tained high. 

This  shows  the  necessity  for  the  thick-walled  muscular 
ventricles  for  the  strong  auriculo-ventricular,  aortic,  and  pulmo- 
nary valves,  and  for  the  strong-walled  elastic  arteries. 

The  resistance  in  the  pulmonary  circuit  is  not  nearly  so 
great  as  that  in  the  systemic  circuit,  and  Jience  the  hlood-pressure 
in  the  pnlmonary  artery  is  only  about  one-third  of  that  ifi  the 
aorta.  The  walls  of  the  pulmonary  arteries  are  hence  not  nearly 
so  thick  as  those  of  the  aorta  and  its  principal  branches,  and  in 
the  case  of  the  ventricles  themselves  the  left  has  very  much 
thicker  walls  than  the  right,  so  that  although  the  internal 
volume  of  the  two  ventricles  is  the  same  as  pointed  out  above, 
the  external  volume  of  the  left  ventricle  greatly  exceeds  that 
of  the  right. 

The  blood  pressure  in  the  arteries  increases  somewhat  at 
each  stroke  of  the  ventricles,  and  falls  back  between  the  strokes ; 


io6  Elementary  Physiology. 

it  probably  equals  on  the  average  in  the  aorta  in  man  the 
pressure  of  a  column  of  mercury  120-140  millimetres  high^  which 
is  nearly  equivalent  to  a  cohmm  of  water  about  6  feet  high. 

The  pressure  falls  in  each  part  of  the  circulatory  system 
proportionately  to  the  resistance  passed.  Since  there  is 
practically  no  resistance  in  the  larger  arteries,  there  is  very 
little  fall  below  that  of  the  aorta  (or  pulmonary  artery  respec- 
tively) until  the  arterioles  are  reached.  The  amount  of 
pressure  (or  head^  as  it  is  called)  lost  in  the  arterioles  is  very 
variable,  according  to  whether  the  muscular  walls  of  these  are 
constricted  or  relaxed,  but  is  always  much  greater  than  that 
lost  in  the  larger  arteries.  In  the  capillaries  a  still  greater 
resistance  is  encountered,  and  a  great  fall  in  pressure  results; 
so  that  when  the  veins  are  reached  nearly  all  the  arterial 
pressure  has  been  dissipated  in  overcoming  resistance.  The 
veins  are  wide  tubes,  like  the  arteries,  and  very  little  pressure 
is  required  to  send  the  blood  along  them  back  to  the  heart ;  so 
that,  although  there  is  a  slow  fall,  the  gradient  is  very  slight. 
Finally,  when  the  auricles  are  again  reached  by  the  returning 
blood,  all  that  pressure  under  which  it  was  sent  forth  on  its 
round  by  the  ventricles  has  become  lost.  In  fact,  each  time 
we  draw  our  breath  the  lungs  are  filled,  as  has  been  previously 
said,  by  the  suction  on  them  of  the  enlarged  thorax,  and  this 
suction  comes  to  bear,  not  only  on  the  lungs,  but  on  all 
distensible  structures  within  the  thorax ;  so  that  there  is  a 
suction  on  the  large  veins  within  the  thorax  as  also  upon  the 
auricles  themselves. ^  Hence,  in  inspiration,  the  pressure  in 
the  large  veins  may  not  only  fall  to  zero,  but  become  negative, 
and  as  this  suction  is  transmitted  to  the  large  veins  immediately 
outside  the  thorax,  such  as  the  jugular  in  the  neck,  if  such  a 
vein  be  inadvertently  cut,  air  may  enter  during  inspiration,  on 
account  of  this  negative  pressure,  and  produce  fatal  results 
when  it  reaches  the  heart. 

^  For  methods  of  measuring  blood  pressure,  the  student  must  consult 
larger  text-books. 

^  Another  effect  of  this  suction  is  to  cause  a  greater  flow  of  blood 
towards  the  thorax,  and  hence  towards  the  heart,  during  inspiration.  This 
increased  amount  of  blood  is  pumped  round  by  the  heart ;  so  that  there  is  a 
small  rise  of  blood  pressure  during  inspiration,  and  a  small  fall  during 
expiration. 


The  Circulatory  System.  loy 

The  arteries,  as  has  been  pointed  out  above,  are  tubes  with 
elastic  ^  walls  capable  of  being  distended  under  pressure  as  the 
blood  is  pumped  into  them  from  the  ventricles,  and  of  recover- 
ing their  former  dimensions  as  it  escapes  into  the  capillaries. 
The  use  of  this  property  is  obvious,  for  if  the  arteries  were  rigid 
tubes  under  pressure,  each  quantity  of  fluid  shot  into  them 
would  produce  a  spurt  passing  all  through  the  vascular  system. 
There  would  be  no  steady  stream  passing  through  the  capillaries 
to  feed  the  tissues,  but  instead  a  useless  onward  spurt  at  each 
stroke,  and  between  the  strokes  no  forward  flow.  In  fact, 
perfectly  rigid  tubes  to  replace  the  arteries  it  is  almost  im- 
possible to  conceive  in  action ;  for  the  capillaries  are  so  narrow 
that  they  would  present  an  enormous  resistance  to  the  flow  of 
blood  through  them  with  such  a  sudden  large  velocity,  and  so 
the  arteries  would  burst,  or  the  heart  stop  dead,  incapable  of 
expelling  its  contents  in  face  of  such  a  resistance.  The  dis- 
tensibility  of  the  arteries  plays  in  the  vascular  mechanism 
exactly  the  same  part  as  the  air-chamber  of  a  pumping  or  fire- 
engine,  or  as  the  indiarubber  cover  of  a  chemical  bellows. 
At  each  heart-beat  the  quantity  of  blood  discharged  from  the 
ventricle  somewhat  further  distends  the  large  arteries  and 
increases  the  pressure  slightly  within  them.  Then,  after  the 
blood  ceases  to  flow  in  (during  the  diastolic  pause),  the  elastic 
walls  of  the  artery  continue  to  press  on  the  blood  and  drive  it 
in  a  steady  stream  onward  into  the  capillaries.  It  is  this  action 
of  the  arterial  walls  which  is  responsible  for  the  difference  in 
character  of  the  flow  from  a  cut  artery  and  that  from  a  cut 
vein  respectively.  When  an  artery  is  cut  there  is  a  rapid  flow 
in  jerks /;-^;;^  the  e?id  nearer  the  heart ;  when  a  vein  is  cut  there 
is  a  slower,  oozing,  and  constant  ^o^  fro7?i  the  end  farther  fro  f?i 
the  heart  and  nearer  to  the  capillaries.  The  difference  in  the 
ends  from  which  the  blood  flows  is  caused  by  the  direction  of 
the  blood-stream ;  the  more  rapid  flow  from  the  artery  is  due 
to  the  higher  pressure  under  which  the  blood  is  within  it ;  and 
the   spurting  flow  from   the   artery  is  due   to  the   heart-beats 

^  The  word  "elastic"  is  here  used  in  the  popular  sense  to  mean  dis- 
tensible by  pressure  and  capable  of  recovery  afterwards,  and  not  in  the 
technical  sense  used  by  physicists. 


io8  Elementary  Physiology. 

rhythmically  increasing  the  pressure;  while  the  constant  flow 
from  the  vein  is  the  result  of  the  uniform  pressure  established 
by  the  interposed  resistance  of  the  capillaries  by  means  of 
which,  combined  with  the  elastic  pressure  of  the  distended 
arteries,  the  stream  is  made  constant. 

It  may  be  gathered  from  the  foregoing  account  that  two 
factors  are  necessary  to  convert  the  pulsatile  flow  of  the  arteries 
into  the  steady  flow  of  the  veins — viz.  first,  the  elastic  arterial 
wall  distended  under  pressure ;  and,  secondly,  the  peripheral 
resistance  of  the  small  channels  (arterioles  and  capillaries) 
interposed  between  the  arteries  and  veins.  When  this  peri- 
pheral resistance  is  much  diminished,  as,  for  example,  when 
the  muscular  walls  of  the  arterioles  are  much  relaxed,  the 
blood  can  pass  too  easily  through  the  arterioles  and  capillaries, 
and  appreciably  more  passes  when  the  pressure  is  increased  at 
a  ventricular  systole,  than  during  ventricular  diastole  :  hence  the 
flow  in  the  veins  beyond  these  distended  arterioles  becomes 
faintly  pulsatile  or  jerky. 

Another  important  outward  physical  sign  of  the  circulation, 
namely  the  pulse^  is  due  also  to  the  elasticity  of  the  arterial 
walls.  When  fluid  is  forced  at  any  point  into  an  elastic  tube 
distended  by  internal  pressure,  a  wave  is  set  up  which  travels 
away  from  this  point  with  a  definite  velocity  depending  upon 
the  elasticity  of  the  wall  and  the  pressure  upon  it.  In  this 
way  an  elastic  wave  is  set  up  in  the  aorta  and  its  branches 
as  each  quantity  of  blood  is  discharged  from  the  left  ventricle 
into  its  upper  end.  This  wave  is  known  as  the  p^Use  wave^ 
and  occasions  the  pulse  felt  when  an  artery  at  any  part 
of  the  body  is  compressed  by  the  finger.  The  pulse  has  the 
same  frequency  as  the  heart-beat,  and  so  is  a  signal  of  the 
rate  and  regularity  with  which  the  heart  is  working.  By  its 
character  it  also  gives  an  indication  of  the  amount  of  pressure 
within  the  artery.  It  must  not  be  supposed  that  the  pulse 
wave  is  directly  due  to  the  motion  of  the  blood  along  the 
artery,  any  more  than  the  waves  of  the  sea  mean  a  motion 
of  the  water  in  the  direction  of  the  waves.  When  a  fresh 
quantity  of  blood  is  thrown  into  the  beginning  of  the  aorta 
there  is  a  distension  of  this  portion  of  the  aorta,  and  afterwards 


The  Circulatory  System.  109 

a  back-swing,  the  distension  is  propagated  along  the  aorta  and 
its  branches,  and  it  is  this  propagated  wave  which  is  the  pulse. 
If  the  pulse  movements  be  magnified,  as  can  be  done  by  an 
instrument  called  a  sphygmograph  (consisting  essentially  of  a  pad 
which  presses  on  the  artery  and  moves  a  lever,  the  longer  end 
of  which  writes  on  a  smoked  paper  surface  moved  past  it  by 
clockwork),  it  is  seen  that  the  pulse  is  not  a  simple  wave,  but 
has  several   secondary   waves  upon  it  (see    Fig.  62).      These 


^■^ 


Fig.  62. — Sphygmographic  tracing. 
The  tracing,  which  is  to  be  read  from  left  to  right,  is  a  record  of  the  pulsations  of  the 
radial  arterj'  in  man.     The  first   strong   upstroke  shows  the  primary  or  percussion 
wave.    The  notch  is  the  "dicrotic  notch,"  and  the  wave  following  it  is  the  "dicrotic 
wave." 

secondary  waves  are  due  to  elastic  after-vibrations  of  the  artery, 
and  one,  called  the  dicrotic  wave.,  is  accentuated  by  a  smaller 
wave  caused  by  the  closure  of  the  semilunar  valves  as  the 
pressure  of  the  blood  in  the  aorta  shuts  them  after  the  ventricle 
has  commenced  to  relax.  This  dicrotic  wave  becomes  mag- 
nified when  the  pressure  in  the  arteries  is  low. 

The  rate  at  which  the  pulse  wave  travels  is  variable,  being 
increased  either  by  greater  rigidity  of  the  arterial  walls  or  by 
increased  arterial  pressure ;  it  is  approximately  3  to  4  metres 
{i.e.  10  to  13  feet)  per  second. 

The  real  velocity  of  the  hlood-stream  along  the  arteries  is 
many  times  less  than  this,  amounting  to  only  30  centimetres 
(about  I  foot)  per  second  in  the  aorta. 

There  is  a  definite  relationship  between  the  velocities  of  the 
blood  in  the  vai'ious  parts  (arteries,  capillaries,  and  veins)  of  the 
vasctilar  system^  which  is  determined  by  one  thing  ojily,  viz. 
the  relative   total  cross-section  ^   of  the  vascular   system   at  the 

^  i.\  cross-section  is  the  area  made  by  a  cut  at  right  angles  to  the  bore 
of  the  tube  (artery,  capillary,  or  vein). 


1 10  Elementary  Physiology. 

given  point.  Each  time  that  an  artery  branches  on  its  way  to 
the  supply  of  a  tissue,  although  the  branches  become  smaller 
their  united  cross-section  becomes  greater.  The  united  cross- 
section  of  the  capillaries  is  many  times  greater  than  that  of  the 
great  arteries  or  veins.  If  the  cross-section  of  the  aorta  be 
taken  as  unity,  then  the  united  cross-section  of  the  capillaries 
is  approximately  500,  and  the  united  cross-section  of  the  ve7i(z 
cavcB  is  about  2  ;  it  follows  that  the  average  velocity  of  the 
blood-flow  in  the  capillaries  is  500  times  slower  than  in  the 
aorta,  and  in  the  vence  cavce  is  about  half  as  fast.  For  the  blood 
must  pass  in  turn,  in  making  the  complete  circuit,  through  the 
entire  vascular  system.  A  quantity  of  blood  flowing  in  a  given 
time  along  the  aorta  must  flow  in  the  same  time  exactly 
through  the  united  capillaries,  and  afterwards  through  the 
vence  cava;.  Now,  the  velocity  of  flow  is  evidently  obtained 
by  dividing  the  quantity  of  blood  flowing  in  the  unit  of  time 
by  the  cross-section  of  the  channel  or  channels  through  which 
it  flows.  If  the  channel  be  of  twice  the  area  the  blood  must 
only  flow  at  half  the  rate  for  the  same  quantity  to  flow  in  the 
same  time.  Hence,  as  the  same  blood  flows  all  round  the  circuit, 
the  velocity  at  any  part  is  inversely  proportional  to  the  total 
cross-section  at  that  part. 

The  velocity  in  the  aorta  being  about  30  centimetres  (about 
I  foot)  per  second,  that  in  the  capillaries  will  roughly  average 
on  the  above  basis  something  over  half  a  millimetre  (~  of  an 
inch)  per  second,  and  that  in  the  great  systemic  veins  about 
15  centimetres  (6  inches)  per  second. 

It  is  often  erroneously  stated  that  the  resistance  in  the 
minute  capillaries  is  a  cause  of  the  slow  flow  of  the  blood  in 
these  vessels  as  compared  with  that  in  the  arteries,  but  this  is 
a  fallacy;  the  comparative  cross-section  is  the  only  factor  in 
determining  the  relative  velocities.  It  is  indeed  true  that  a 
diminution  of  the  resistance  in  the  capillaries,  or  rather  in  the 
small  arterioles  which  lead  to  them,  will  increase  the  flow 
through  these  capillaries,  but  it  will  proportionately  increase  the 
velocity  of  flow  in  the  arteries  and  veins,  if  the  diminution  in 
capillary  resistance  be  general  all  over  the  body,  and  hence  the 
relative  velocity  will  remain  unchanged.     In  this  manner,  any 


Tlie  Circulatory  System.  1 1 1 

general  change  in  velocity  at  any  part  of  the  circuit  must  tell 
backward  and  forward  on  the  velocity  in  all  other  parts  of 
the  circuit,  and  the  average  velocity  in  arteries,  veins,  and 
capillaries  must  remain  purely  determined  by  the  relative  total 
cross-section  at  these  various  parts. 

The  local  velocity  through  the  capillaries  of  a  given  area — 
say  of  the  capillaries  of  any  particular  organ  in  the  body — is, 
however,  much  more  important  than  the  average  capillary 
velocity,  and  this  can  be  altered  very  widely,  without  much 
altering  the  velocity  in  the  arteries  or  veins,  by  means  of 
variations  in  the  calibre  of  the  arterioles  supplying  the  part. 
These  variations  are  brought  about  by  nerve  action  through 
nerve  fibres  supplied  to  the  involuntary  muscle  coat  of  these 
arterioles — the  vaso-motor  fibres.  By  this  means  the  distribution 
of  the  blood-stream  to  the  different  organs  is  regulated  ;  almost 
shut  off  w^hen  the  demand  is  small,  through  the  organ  becoming- 
dormant,  and  turned  on  in  full  when  the  organ  again  passes 
into  a  state  of  activity.  The  vaso-motor  nerve  fibres,  and  the 
involuntary  muscle  coat  of  the  arterioles  on  which  they  act, 
are  thus  to  the  circulation  of  the  blood  what  the  distributing 
taps  are  to  a  water-supply,  allowing  the  stream  to  flow  where 
there  is  work  for  it  to  do,  and  shutting  it  off  where  it  is  not 
required.  The  only  difference  is  that,  in  the  body,  the  taps 
are  never  completely  shut  down,  they  are  only  more  or  less 
widely  opened.  The  reason  for  this  is,  that  living  tissue  cannot 
go  on  for  a  long  period  without  a  supply  of  oxygen  (which  is 
carried  to  it  by  the  blood-stream).  Even  in  a  resting  condition 
a  certain  amount  of  activity  goes  on,  accompanied  by  oxidation 
and  need  for  respiration.  But  when  the  cells  become  active, 
the  amount  of  change  becomes  largely  increased,  and  the  blood- 
supply  must  also  be  increased  to  cope  wdth  the  larger  demand. 

In  certain  veins,  particularly  in  those  of  the  limbs,  the 
action  of  the  heart  is  assisted  by  valves  placed  in  the  walls  at 
intervals.  These  ve?io7is  valves  are  very  simple  structures,  con- 
sisting of  two  pouch-like  invaginations  of  the  inner  coat  of  the 
vein  placed  opposite  each  other  (see  Fig.  6t,).  They  open 
towards  the  heart,  and  allow  of  the  passage  of  the  blood  in 
that  direction,  but  close  and  prevent  any  flow  in  the  opposite 


112 


Elementary  Physiology. 


Fig.  63. — Diagram  showing  the 
valves  of  veins. 


direction.  The  use  of  these  valves  is  in  determining  the  direc- 
tion of  flow  when  a  vein  is  compressed  by  muscular  action  in 
moving  the  part.  Were  there  no  valves,  the  contents  of  the 
vein  on  compression  would  move  in  both   directions  to  and 

from  the  heart,  so  as  to  empty  the 
compressed  portion,  for  the  venous 
pressure  (5-15  millimetres  of  mer- 
cury) is  too  small  to  prevent  this 
action.  But  the  valves  have  the 
effect  of  causing  the  vein  to  empty 
its  contents  towards  the  heart  only, 
and  so  assist  the  heart  in  its  work. 
This  action  is  best  seen  in  the  long 
veins  of  the  leg,  which  are  plentifully 
supplied  with  valves.  If  a  person 
remain    perfectly    at   rest    for    some 

A,  part  of  a  vein  laid  open,  with       .  .  ... 

two  pairs  of  valves ;  B,  longi-    time  in  an  Upright  position,  the  veins 

tudinal  section  of  a  vein,  show-        ..11  -      r  ^-l     i  i.        i.  i.t_„ 

in-  the  valves  closed ;  c,  por-  at  the  lowcr  part  of  the  leg,  about  the 
:XibiSng\swSgltatir  anklc,  bccoiTie  Considerably  swollcn, 
of  valves.  bccause  the  venous  pressure  is  thus 

greatly  increased,  and  the  veins  become  rounded  and  distended 
by  the  increased  pressure.  The  pressure  in  the  veins  must, 
under  such  conditions,  be  equal  to  the  hydrostatic  pressure  of 
a  column  of  blood  reaching  up  to  the  heart  before  the  blood 
can  move  upward  to  the  heart  along  these  veins,  and  this  is  a 
considerable  pressure.^  But  let  the  person  now  make  a  few 
muscular  movements  of  the  legs,  such  as  by  walking  about 
vigorously,  and  the  distension  becomes  greatly  diminished, 
because  the  valves  now  come  into  action,  and  the  blood  is 
pumped  upwards  by  their  action,  as  the  skin  compresses  them 
on  account  of  the  movement  of  the  underlying  muscles. 

The  action  of  the  venous  valves  may  be  tested  by  com- 
pressing with  the  finger  a  long  superficial  vein  of  the  arm  or 

^  It  is  probably  impossible  to  stand  so  still  that  there  is  no  action  at  all 
of  these  valves.  It  may  also  be  vvxll  to  point  out  that  the  heart  has  not  to 
do  work  against  the  hydrostatic  venous  pressure  above  mentioned,  since  it 
is  balanced  by  an  equal  column  in  the  arteries,  and  that  the  chief  evil  effect 
is  the  distension  of  the  veins,  which  is  relieved  by  the  action  of  the  valves 
when  there  is  sufficient  muscular  action. 


The  Circulatory  System.  1 1 3 

leg.'^  If  the  vein  be  stroked  towards  the  heart,  the  blood  runs 
easily  in  front  of  the  finger  and  leavts  the  vein  almost  empty 
behind  it ;  but,  if  it  be  stroked  in  the  opposite  direction,  the 
vein  swells  up,  and  becomes  knotted  at  points  W'here  the  valves 
are  situated.^ 

Structure  of   the   Heart. 

We  may  now  consider  more  in  detail  the  mechanism  of 
the  heart-pump,  and  the  arrangement  of  its  valves.  The 
description  can  only  be  followed  profitably  if  accompanied  by 
a  dissection  of  a  heart.  A  sheep's  heart  can  easily  be  pro- 
cured from  a  butcher,  and  should  be  obtained  with  the  attached 
vessels  as  long  as  possible,  and  uninjured  by  knife-cuts. 

In  the  dead  heart  by  far  the  greater  part  of  the  bulk  is 
made  up  of  the  two  ventricles.  The  auricles  are  two  com- 
paratively inconspicuous  appendages  of  a  deep  purple  colour 
placed  above  the  ventricles.  This  is,  in  part,  due  to  the  fact 
that  even  the  internal  capacity  of  the  auricles  when  distended 
with  blood  is  somewhat  less  than  that  of  the  ventricles,  and  in 
part  to  the  fact  that  the  auricles  have  thin  and  the  ventricles 
thick  walls,  so  that  the  external  volume  of  the  ventricles  is 
much  greater  than  that  of  the  auricles. 

There  is  a  deep  transverse  groove  round  the  heart,  the 
auriculo-ventncular  groove^  separating  the  auricles  from  the 
ventricles.  Another  somew^hat  U-shaped  groove,  called  the 
inter-ventriaUar  groove,  separates  the  two  ventricles ;  it  con- 
tains blood-vessels  and  some  fat,  which  serve  to  more  clearly 
mark  its  position.  This  groove  passes  near,  but  not  over,  the 
apex  of  the  heart,  for  the  entire  apex  belongs  to  the  left 
ventricle,  which  serves  as  a  sign  to  identify  it  in  the  excised 
heart.     Even  before  making  any  incision  into  the  heart,  the 

*  Some  veins  have  no  valves,  for  example,  the  veiut:  cava,  the  pul- 
monary veins,  and  the  veins  of  the  liver  (portal  and  hepatic  veins). 

-  This  experiment  can  be  best  carried  out  on  another  person  with  pro- 
minent veins.  Compress  a  long  vein  of  the  arm  with  one  forefinger,  and 
then  with  the  other  forefinger  stroke  it  upwards  towards  the  heart  ;  the  vein 
empties,  and  remains  empty  for  a  certain  distance  up,  where  a  prominent 
valve  appears.  On  releasing  the  vein,  it  fills  from  the  peripheral  end.  If 
the  experiment  does  not  succeed  with  one  vein  try  another, 

I 


114  Elementary  Physiology. 

thinner  wall  of  the  right  ventricle,  as  compared  with  the  left, 
may  be  appreciated  by  pressure  with  the  fingers. 

When  the  heart  is  in  position  in  the  body,  the  right  side 
lies  more  anteriorly  than  the  left ;  so  that  when  seen  from  the 
front,  two-thirds  is  formed  by  the  right  ventricle,  and  only  the 
third  on  the  left  is  left  ventricle. 

Each  auricle  is  prolonged  somewhat  at  one  side  into  a 
process  resembling  the  lobe  of  the  ear,  and  hence  termed  the 
auriciUar  ^  appendage.  The  main  part  of  each  auricle  is  termed 
the  atrium^  or  sinus  venosus^  because  the  veins  open  into  this  part. 

The  great  vessels  entering  the  base  of  the  heart  should  next 
be  examined. 

The  aorta  is  distinguished  by  having  the  thickest  wall, 
and  by  the  fact  that  the  little  finger  when  inserted  into,  it, 
can  be  passed  into  the  left  ventricle  without  first  passing 
through  the  left  auricle.  The  pulmonary  artery  has  the  next 
thickest  wall,  and  the  finger  when  passed  through  it  passes 
directly  into  the  right  ventricle.  The  remaining  vessels  are 
veins  \  two  enter  the  right  auricle — these  are  the  s2penor  and 
inferior  vencB  cavoR.  The  veins  entering  the  left  auricle  come 
from  the  lungs,  and  are  termed  the  pulmonary  veins.  Their 
number  is  variable  in  different  animals.  In  the  sheep  there  are 
usually  two  j  in  man  there  are  four,  two  from  each  lung. 

The  interior  of  the  heart-chambers  and  the  arrangement  of 
the  valves  should  next  be  studied,  and  it  will  be  well  in  doing 
so  to  follow  the  course  of  the  blood  through  the  heart. ^ 

To  expose  the  inner  surface  of  the  right  auricle,  make  a 
cut  from  the  superior  to  the  inferior  vejia  cava,  and  another 
running  nearly  at  right  angles  from  the  middle  of  this  into  the 
auricular  appendix.  The  inner  wall  shows  muscular  ridges 
over  the  appendix  and  upon  the  right  side  of  the  atrium,  while 
the  rest  of  the  wall  is  much  smoother.  On  the  inter-auricular 
septum,  separating  this  chamber  from  the  left  auricle,  an  oval 
depression  may  be  seen,  called  the  fossa  ovalis  (see  Fig.  64) ; 

^  The  whole  auricle  gets  its  name  from  a  fancied  resemblance  to  the 
external  ear. 

-  Before  making  the  incisions  above  described,  the  student  should,  if 
provided  with  the  necessary  apparatus,  perform  the  experiment  described 
in  practical  exercises,  to  demonstrate  the  action  of  the  valves. 


TJic  Circulatory  System.  1 1 5 

this  corresponds  to  a  former  opening  between  the  two  auricles 
which  existed  in  the  fcetus,  and  served  an  important  purpose 
before  the  lungs  came  into  use,  in  directing  the  blood  into 
the  left  auricle  directly,  instead  of  via  the  lungs  ;  it  became 
closed  later,  as  the  lungs  came  into  use  after  birth,  by  a  fold  of 
tissue,  which  originally  served  as  a  valve  for  it. 

At  this  stage  in  the  dissection,  if  the  auricle  be  held  open 
and  the  water  be  poured  into  it  from  a  slight  height,  so  as  to 
pass  into  the  ventricle,  the  action  of  the  auriculo-ventricular 
valve  may  be  demonstrated.  As  the  water  fills  the  ventricle, 
the  flaps  of  the  valve  float  up  and  close  the  opening.  This 
resembles  what  happens  during  life,  when  the  auricle  contracts 
and  forces  its  contents  into  the  already  ahnost  full  ventricle. 
The  valve-flaps  then  float  up,  and  their  edges  become  opposed, 
so  that  all  is  ready  for  the  ventricular  systole,  and  there  is 
no  back-flow  into  the  auricle  at  the  beginning  of  the  stroke. 

Observe,  further,  that  there  are  no  valves  at  the  openings  of 
the  verm  cavce  into  the  auricle.  Such  valves,  as  has  already 
been  stated,  are  unnecessary,  because  there  is  no  great  force 
exerted  by  the  auricular  contraction,  which  simply  serves  to 
completely  fill  the  ventricle.^ 

Next  open  the  right  ventricle  by  making  an  incision  with 
a  knife  into  it  parallel  to  the  inter-ventricular  groove,  and 
following  this  round  at  some  distance  from  the  groove  so  as 
not  to  injure  the  inter-ventricular  septum.'^  By  means  of  this 
U-shaped  incision  a  flap  is  made  which  can  be  turned  outward 
from  the  apex,  and  so  the  interior  of  the  ventricle  be  examined. 
Feel  with  your  finger,  from  the  inside  of  the  ventricle,  for  the 
opening  of  the  pulmonary  artery  into  the  ventricle,  and  make 
certain  that  you  have  found  it  by  seeing  that  you  come  out  at 
the  external  opening  of  the  pulmonary  artery  which  you  have 

•  Back-flow  into  these  veins  is  also  prevented  by  the  facts,  that  there  is 
Httle  resistance  to  the  discharge  into  tlie  ventricle,  that  there  is  already  a 
current  of  blood  towards  the  auricle  in  the  veins,  and  that  the  veins  assist 
the  process  by  a  slight  contraction  of  their  walls  immediately  preceding 
that  of  the  auricle. 

*  This  is  the  muscular  septum  separating  the  two  ventricles.  It  is 
somewhat  convex  towards  the  right  ventricle  ;  so  that  the  right  ventricle 
is  crescentic  in  cross  section,  while  the  left  ventricle  in  cross  section  is  oval 
or  rounded. 


Ii6  Elementary  Physiology. 

previously  identified  as  above  described.  Now,  using  your 
finger  as  a  guide,  continue  the  previous  cut  in  the  ventricle  up 
towards  the  pulmonary  artery,  and  cut  through  the  lower 
portion  of  the  wall  of  this,  if  possible  between  two  of  the  three 
flaps  of  the  semilunar  valve  which  guards  its  orifice.  Finally, 
cut  away  the  lower  part  of  the  flap  of  ventricular  wall  already 
partially  detached,  so  as  to  expose  the  interior  of  the  ventricle 
more  completely. 

The  inner  surface  of  the  ventricle  is  ridged  by  strong 
muscular  bundles  (the  columncE  carnece)^  some  of  which  are 
attached  to  the  wall  of  the  ventricle  all  the  way,  others  are  free 
in  the  middle,  but  attached  at  both  ends  {trabeailcd)^  while 
others,  again,  are  attached  to  the  ventricular  wall  only  at  their 
base  {imisadi  papillares).  This  last  set  (musculi  papillares), 
which  form  two  chief  bundles,  anterior  and  posterior,  serve 
as  contractile  pillars  for  the  attachment  of  strong  fibrous  cords 
(the  chordcs  tendinece)^  which  are  fixed  by  their  other  ends  to 
the  edges  and  under-surfaces  of  the  flaps  of  the  tricuspid  valve. 
The  tricuspid  valve  (see  Fig.  64)  guards  the  passage  from  the 
.right  auricle  to  the  right  ventricle,  and  makes  this  opening 
impermeable  when  the  ventricle  contracts.  It  consists  of  three 
thin  but  strong  fibrous  flaps,  roughly  triangular  in  form,  with 
their  bases  attached  in  a  complete  ring  round  the  auriculo- 
ventricular  orifice,  and  their  apices  towards  the  ventricle.  To 
the  edges  of  the  flaps,  and  to  their  under-surfaces,  the  strong 
thin  ckordcB  tefidinece  above  mentioned  are  attached,  and  hold 
the  flaps  in  position  so  as  to  prevent  them  bulging  into  the 
auricle  when  the  ventricle  contracts  and  the  blood  presses  upon 
their  under  surfaces. 

The  musculi  papillares  are  a  compensating  arrangement.  If 
the  cords  by  which  the  flaps  are  held  down  were  non-contractile, 
and  ran  straight  from  the  valve  to  the  ventricular  wall,  then 
the  wall  would  move  towards  the  valve  as  the  ventricle  con- 
tracted, the  ends  of  the  restraining  cords  would  move  with  it, 
the  flaps  would  be  allowed  to  move  too  far  up,  and  the  valve 
become  incompetent.  This  is  prevented  by  the  contractile 
musculi  papillares,  which  shorten  as  the  ventricle  contracts, 
and  so  hold  the  valves  in  the  proper  position. 


TJie  Circidatory  System. 


117 


The  wall  of  the  ventricle  near  the  mouth  of  the  pulmonary 
artery  is  smooth,  and  conical  in  shape  {conns  arteriosus)^  narrow- 


FiG.  64. — Interior  of  the  right  auricle  and  ventricle,  exposed  by  removal  of  the  greater 
part  of  their  right  and  anterior  walls.     (Allen  Thomson.)     \ 

I,  superior  vena  cava  ;  2.  inferior  vena  cava  ;  2',  hepatic  veins  ;  3,  septum  of  the  auricles ; 
3',  fossa  ovalis  ;  the  Eustachian  valve  is  just  below  ;  3",  aperture  of  the  coronary  sinus 
with  its  valve;  +,  +,  right  auriculo-ventricular  groove,  a  narrow  portion  of  the 
adjacent  walls  of  the  auricle  and  ventricle  having  been  preserved  ;  4,  4,  on  the  septum, 
the  cavity  of  the  right  ventricle  ;  4',  large  anterior  papillary  muscle  ;  5,  infundibular, 
5',  right,  and  5",  posterior  or  septal  segment  of  the  tricuspid  valve  ;  6,  pulmonaiy 
artery,  a  part  of  the  anterior  wall  of  that  vessel  having  been  removed,  and  a  narrow 
portion  of  it  preserved  at  its  commencement  where  the  pulmonary  valve  is  attached  ; 
7,  the  aortic  arch  close  to  the  cord  of  the  ductus  arteriosus  ;  8,  ascending  aorta 
covered  at  its  commencement  by  the  auricular  appendix  and  pulmonary  artery  ; 
9,  placed  between  the  innominate  and  left  common  carotid  arteries  ;  10,  appendix  of 
the  left  auricle  ;   11,11,  left  ventricle. 

ing  towards  the  artery.     The  mouth  of  the  artery  is  circular, 
and  about  an  inch  in  diameter ;  it  occupies  the  summit  of  the 


ii8 


Elementary  Physiology. 


Fig.  65. 


-The  left  auricle  and  ventricle  opened  and  a  part  of  the  wall  removed  so  as  to 
show  their  interior.     (Allen  Thomson.)    ■? 


The  commencement  of  the  pulmonary  artery  has  been  cut  away,  so  as  to  show  the  aorta  ; 
the  opening  into  the  left  ventricle  has  been  carried  a  short  distance  into  the  aorta 
between  two  of  the  semilunar  flaps ;  and  part  of  the  auricle  with  its  appendix  has 
been  removed,  i,  right  pulmonary  veins  cut  short ;  i',  placed  within  the  cavity  of 
the  auricle  on  the  left  side  of  the  septum,  on  the  part  formed  by  the  valve  of  the 
foramen  ovale,  of  which  the  crescentric  border  is  seen  ;  2',  a  narrow  portion  of  the 
wall  of  the  auricle  and  ventricle  preserved  around  the  auriculo-ventricular  orifice  ; 
3,  3',  cut  surface  of  the  wall  of  the  ventricle,  seen  to  become  very  much  thinner 
towards  3",  at  the  apex  ;  4,  a  small  part  of  the  wall  of  the  left  ventricle  which  has 
been  preserved  with  the  left  papillary  muscle  attached  to  it ;  5,  5,  right  papillary 
muscles ;  5',  the  left  side  of  the  septum  ventriculorum ;  6,  the  anterior  or  aortic 
segment,  and  6',  the  posterior  or  parietal  segment  of  the  mitral  valve  ;  7,  placed  in 
the  interior  of  the  aorta  near  its  commencement  and  above  its  valve  ;  7',  the  exterior 
of  the  great  aortic  sinus  ;  8,  the  upper  part  of  the  conus  arteriosus  with  the  root  of 
pulmonary  artery  and  its  valve  ;  8',  the  separated  portion  of  the  pulmonary  trunk 
remaining  attached  to  the  aorta  by  9,  the  cord  of  the  ductss  arteriosus  ;  10,  the 
arteries  arising  from  the  aortic  arch. 


The  Circulatory  System.  1 1 9 

ventricle,  being  situated  slightly  higher  than  the  auriculo- 
ventricular  opening,  and  is  guarded  by  a  valve  with  three 
pouch-like  (semilunar)  flaps  (see  Fig.  66),  called  \kv^  pulmonary 
scmiliuiar  valve ^  which  opens  towards  the  artery. 

The  construction  on  the  left  side  of  the  heart  is  very  similar 
to  that  on  the  right,  except  that  everything  is  more  strongly 


Fig.  (>(>. — View  of  the  base  of  the  ventricular  part  of  the  heart,  showing  the  relative 
position  of  the  arterial  and  auriculo-ventricular  orifices.     (Allen  Thomson.;     \ 

The  muscular  fibres  of  the  ventricles  are  exposed  by  the  removal  of  the  pericardium,  fat, 
blood-vessels,  etc  ;  the  pulmonary  artery  and  aorta  and  the  auricles  have  been 
removed  ;  the  valves  are  in  the  closed  condition,  i,  i,  right  ventricle  ;  i',  conus 
arteriosus  ;  2,  2,  left  venticle  ;  3,  3,  the  divided  wall  of  the  right  auricle  ;  4,  that  of 
the  left  ;  5,  the  inlundibular  ;  5',  the  right,  and  5",  the  septal  segment  ot  the  tricuspid 
valve  ;  6,  the  anterior  or  aortic,  and  6',  the  posterior  or  parietal  segment  of  the 
mitral  valve  (in  the  angles  between  these  segments  are  seen  smaller  lobes) ;  7,  the 
pulmonary  artery ;  8,  placed  upon  the  root  of  the  aorta;  9,  the  right;  9',  the  left 
coronary  artery. 

fashioned  in  the  ventricle  on  account  of  the  greater  pressure  to 
be  overcome  in  the  aorta. 

Four  pulmonary  veins  enter  the  left  auricle  in  man  {I'ide 
supra),  two  from  each  lung  opening  close  together  into  opposite 
sides  of  the  cavity.  To  open  the  left  auricle  a  cut  should  be 
made  across  the  posterior  surface  from  the  right  to  the  left  pul- 
monary veins,  and  another  shorter  cut  towards  the  front  at  right 
angles  to  this  one.  The  mitral  valve  guarding  the  left  auriculo- 
ventricular  opening  may  next  be  floated  up  by  pouring  in  water 
in  a  similar  fashion  to  that  directed  above  in  the  case  of  the 
tricuspid  valve.     //  has  only  two  flaps  instead  of  three,  but  is 


I20  Elementary  Physiology. 

much  stronger  than  the  tricuspid  valve  in  its  structure ;  other- 
wise the  construction  of  the  two  valves  is  much  the  same. 

The  left  ventricle  may  be  opened  by  an  incision  passing 
along  both  anterior  and  posterior  surfaces  parallel  and  close  to 
the   inter-ventricular   septum.     When   the   flap   so   formed   is 
turned  out,  the  interior  of  the  ventricle  is  seen.     The  cavity  is 
longer  and  more  conical  in  shape  than  that  of  the  right  ven- 
tricle.    Similarly  to  the  right  ventricle  it  has  two  orifices,  one 
leadingy9v;;^  the  auricle,  the  other  mto  the  aorta.     The  walls  are 
also  roughened  by  muscular  projections,  except  near  the  mouth 
of  the  aorta  where  they  are  smoother.     The  musculi  papillares 
of  the  left  ventricle,  giving  attachment  to  the  chordcz  tendinece 
at  one  end,  are  strongly  developed,  and  arranged  in  two  large 
bundles  which  spring  respectively  from  the  right  and  left  sides 
of  the  cavity.     The  aortic  opening  is  situated  somewhat  higher 
up   and  more  to  the  front  than  the  auriculo- ventricular,  and 
both  these  orifices  are  slightly  narrower  than  those  on  the  right 
side.     The  entrance  to  the  aorta  is  guarded  by  the  aoi^tic  semi- 
lunar valve,  which  like  that  at  the  pulmonary  artery  has  three 
pouch-like  flaps,  but  is  of  stronger  construction.     The  pouches 
in  each  case  are  folds  of  the  inner  coat  of  the  artery  strengthened 
by  strong  fibrous  tissue.     Where  the  three  flaps  meet  in  the 
closed  position  of  the  valve  there  are  small  nodules  of  cartilage 
called  the  corpora  Arantii.     Opposite  each  pouch  of  the  valve 
there  is  a  swelling  outwards  of  the  wall  of  the  vessel  known 
as  the  siims  of  Valsalva,  and  at  the  upper  margins  of  two  of 
these  sinuses,  two  openings  are  situated,  which  are  the  orifices 
of  the  coronary  arteries.     These  arteries,  the  first  branches  of 
the  aorta,  supply  the  heart  itself  with  blood.     The  veins  of  the 
heart  open  into  the  right  auricle  by  various  openings,  but  chiefly 
by  the  coronary  sinus,  situated  between  the  inferior  cava  and 
the  auriculo-ventricular  opening.    This  course  via  the  coronary 
artery,  heart   capillaries,  and    coronary   sinus  is   the    shortest 
circuit  that  the  blood  can  take  in  the  systemic  circulation ;  while 
that  to  the  capillaries  of  the  intestine,  from  these  to  the  capil- 
laries of  the  liver  by  the  portal  vein,  and  back  to  the  inferior 
cava  by  the  hepatic  vein,  is  the  longest  route  which  can  be  taken. 


CHAPTER   VI. 

THE  BLOOD. 

The  nutrient  fluid  which  circulates  in  the  blood-vessels  varies 
in  colour  from  bright  red  to  dark  purple,  according  to  the 
amount  of  oxygen  which  it  contains.-^  It  is  very  opaque,  even 
in  thin  layers,  and  its  opacity  is  due  to  the  same  cause  as  makes 
clouds  or  sea  foam  opaque  though  made  up  of  transparent 
material,  namely,  that  it  contains  floating  in  it  myriads  of 
minute  particles  (called  corpiLscles)  which  reflect  and  refract 
the  light  in  all  directions  and  refuse  to  allow  it  any  passage  in 
unbroken  lines.  A  small  drop  of  blood  drawn  from  the  finger 
ought  to  be  examined  by  the  student  under  the  high  power  of 
a  good  microscope,  preferably  after  diluting  it  with  about  its 
own  volume  of  physiologically  normal  saline,"-^  for  the  corpuscles 
in  the  blood  are  so  numerous  that  they  cannot  be  quite  so 
clearly  seen  in  undiluted  blood. 

A  large  number  of  small  round  discs  are  seen,  which  are 
occasionally  discovered  rolled  over  on  their  edge,  w^hen  they 
are  seen  to  be  bi-concave  in  outline  (see  r.  Fig.  67).  They  have 
a  tendency  to  adhere  by  their  concave  surfaces,  when  these  come 
in  contact,  so  as  to  form  long  rouleaux.     In  colour  they  are  a 

*  The  bright  red  blood  contains  more  oxygen,  and  is  seen  when  an 
artery  is  cut  through  ;  since  it  is  contained  in  the  arteries  it  is  called  arterial 
blood.  The  dark  purple  colour  is  acquired  as  the  oxygen  disappears  from  the 
blood,  which  it  does  in  the  passage  through  the  tissues,  and  hence  the  veins 
contain  such  blood,  which  is  accordingly  termed  venous  blood. 

-  This  solution  is  easily  made  by  dissolving  6 "5  grams  of  common  salt  in 
a  litre  (176  pint)  of  tap  water  ;  for  frog's  blood,  which  is  also  well  worth 
examining,  it  must  be  stronger  (8  grams  per  litre).  This  solution  has 
approximately  the  same  strength  in  salts  as  the  blood,  and  prevents  altera- 
tions in  shape  of  the  corpuscles  ;  water  makes  them  swell  out,  and  stronger 
salt  solutions  draw  the  water  out  of  them  and  make  them  become  shrunken 
and  crenated. 


122 


Elementary  Physiology. 


pale  yellow,  for  it  is  only  when  seen  e7i  masse  that  the  colouring 
matter  {hamoglobiii)  with  which  they  are  charged  has  a  red  hue. 
There  are  immense  numbers  of  these  red  blood  corpuscles,  or 
blood  discs,  in  the  blood,  the  average  number  being  five  to 
six  millions  per  cubic  millimetre,^  but  in  anaemia  the  number 
may  be  much  reduced. 

There  are,  in  addition  to  the  red  corpuscles,  a  much  smaller 


/ 


Fig.  6S. — Human  red  cor- 
puscles Ij'ing  singly  and 
collected  into  rolls.  (As 
seen  under  an  ordinary- 
high  power  of  the  micro- 
scope.) 
I,  on  the  flat  ;  2,  in  profile. 


Fig.  67.— Human  blood  as  seen  on  the  warm 
stage.     (Magnified  about  1200  diameters.) 

r,  r,  single  red  corpuscles  seen  lying  flat ;  r',  r', 
red  corpuscles  on  their  edge  and  viewed  in 
profile ;  r",  red  corpuscles  arranged  in  rou- 
leaux ;  c,  c,  crenate  red  corpuscles  ;  /,  a 
finely  granular  pale  corpuscle  ;  g,  a  coarsely 
granular  pale  corpuscle.  Both  have  two_  or 
three  distinct  vacuoles,  and  were  undergoing 
changes  of  shape  at  the  moment  of  observa- 
tion ;  in^,  a  nucleus  also  is  visible. 


number  of  luhite  corpuscles^  or  leucocytes.  There  is,  on  an 
average,  one  of  these  white  corpuscles  to  every  four  hundred 
of  the  red  corpuscles  ;  they  are  usually  considerably  larger,  in 
human  blood,  than  the  red  corpuscles,  although  they  vary 
greatly  both  in  size  and  appearance.  Each  possesses  one  or  more 
nuclei ;  some  are  vacuolated,  and  others  granular,  while  others 

^  A  cubic  millimetre  has  about  the  volume  of  a  large  pin's  head. 


TJie  Blood.  123 

again  are  clear  or  nearly  hyaline.  They  exhibit  those  amoeboid 
movements  which  have  been  described  in  the  introduction  to 
this  book,  especially  when  wanned,  and  if  the  blood  be  mixed 
with  fine  particles,  such  as  yeast  cells,  the  white  corpuscles  may 
be  observed  "with  the  microscope  taking  these  into  their  mass. 
This  inception  of  foreign  particles  indicates  an  important  function 
of  the  white  blood  corpuscles.  In  a  similar  manner  they 
absorb  and  render  innocuous  any  deleterious  particles  which 
may  have  found  their  way  in  any  manner  into  the  blood.  They 
congregate  round  a  wound  or  other  infected  part,  and  prevent 
injurious  products  from  entering  the  blood.  In  the  combat 
many  of  the  leucocytes  themselves  become  poisoned  and  die, 
so  that  they  are  found  in  large  number  in  \k\ftpus  flowing  from 
a  suppurating  wound. 

The  leucocytes  are  probably  formed  in  the  lymphatic  glands, 
and  enter  the  blood-stream  with  the  lymph,  for  the  number  of 
leucocytes  in  the  lymph  is  much  increased  after  it  has  flowed 
through  a  lymphatic  gland ;  also,  the  leucocytes  of  lymph  have 
a  preponderance  of  smaller  and  clearer  cells,  which  are  termed 
lymphocytes,  and  have  the  appearance  of  young  leucocytes. 

The  purpose  of  the  red  blood  coipuscles  is  to  absorb  oxygen 
in  passing  through  the  lungs  and  give  it  up  again  in  passing 
through  the  tissues.  About  ninety  per  cent,  of  their  dried 
weight  consists  of  a  complex  substance  of  a  proteid  nature,^ 
called  hcemoglobin^  which  is  capable  of  forming  an  unstable 
chemical  compound  with  oxygen.  This  compound  is  formed 
when  the  pressure  of  oxygen  in  the  fluid  containing  the 
haemoglobin  reaches  a  certain  value,  and  is  decomposed  again 
when  the  oxygen  pressure  falls.  Now,  in  the  tissues,  oxygen 
is  being  constantly  used  up  by  the  oxidations  going  on  there, 
and  consequently  there  is  a  low  oxygen  pressure  ;  while  in  the 
lungs,  by  the  process  of  respiration,  the  oxygen  pressure  is  kept 
fairly  high.     Accordingly,  the  haemoglobin  takes  up  oxygen  in 

^  The  student  is  advised  to  read  the  part  of  chapter  vii.  on  the  chemistry 
of  proteid,  before  reading  this  chapter. 

-  The  name  haemoglobin  is  used  in  a  general  sense  ;  when  combined 
with  oxygen  it  is  often  termed  oxy-hKmoglobin,  and  when  free  of  oxygen 
so  combined  it  is  termed  reduced  haemoglobin.  In  addition  to  the  usual 
proteid  constituents  it  contains  iron. 


124  Elementary  Physiology. 

the  lungs,  and  gives  it  up  again  in  the  tissues.  Thus,  a  supply 
of  oxygen  is  carried  by  the  blood-stream  to  all  parts  of  the 
body  to  serve  in  oxidizing,  in  the  life  processes  of  the  cells, 
those  nutrient  materials  which  the  blood  also  carries  to  the 
tissues. 

The  haemoglobin  can  be  set  free  from  the  corpuscles  in 
various  ways,  such  as,  often  alternately  freezing  and  thawing, 
adding  excess  of  water,  or  a  trace  of  ether  or  chloroform.  It 
then  dissolves  in  the  water  of  the  blood  and  forms  a  transparent 
fluid  of  a  deep  lake  colour  known  as  laked  blood. 

The  corpuscles  of  man  and  of  the  mammalia  generally  are, 
as  stated  above,  non-nucleated  bi-concave  discs ;  but  those  of 
lower  vertebrae  (such  as  amphibia,  fishes,  and  birds)  are  oval 
bodies  with  prominent  nuclei.  Nucleated  fed  blood  corpuscles 
are,  however^  foiDid  before  birth,  even  in  the  mainmaUa.  Red 
blood  corpuscles  are  probably  formed  in  large  numbers,  during 
life,  in  the  red  marrow  of  the  ribs  and  of  the  heads  of  the  long 
bones,  and,  as  before  stated,  it  is  probable  that  they  are,  to 
some  extent,  destroyed  in  the  spleen.  The  liver  also  probably 
takes  a  share  in  the  disintegration  of  effete  red  blood  cor- 
puscles, for  the  bile  pigments  excreted  by  it  are  closely  allied 
to  haemoglobin  in  their  chemical  constitution  (see  p.  164). 

The  corpuscles  float  in  a  clear  fluid,  which  is  of  a  pale  straw 
colour  when  it  is  obtained  uncontaminated  by  haemoglobin 
shed  from  the  red  corpuscles,  and  is  called  the  plasma,  or  blood 
plasma. 

Special  precautions  must  be  taken  in  order  to  obtain  plasma, 
because  it  possesses  the  property  of  coagulating,  or  setting  into  a 
solid  mass.  If  this  coagulation  takes  place  in  shed  blood  before 
the  corpuscles  have  had  time  to  subside  and  separate,  a  thick 
solid  mass  of  blood,  known  as  a  blood-clot,  is  the  result.  The 
power  of  the  plasma  to  undergo  spontaneous  coagulation  after 
the  blood  is  shed,  and  so  to  cause  the  blood  to  set  solid,  is  a 
most  invaluable  quality ;  for,  without  it,  an  animal  would  slowly 
bleed  to  death  even  from  an  insignificant  wound.  By  its  aid, 
however,  the  apertures  of  the  wounded  vessels  are  stopped, 
and  the  leakage  of  blood  ceases. 

As  will  be  presently  pointed  out,  the  coagulation  of  the 


The  Blood.  125 

blood  is  due  to  a  proteid  dissolved  in  the  plasma,  called 
fibrinogen.^  which,  under  certain  conditions,  gives  rise  to  an 
insoluble  substance  (also  a  proteid)  called  fibrin.  The  fibrin 
so  formed  when  blood  clots  is  but  an  insignificant  part  of  the 
blood,  amounting  to  only,  about  two  parts  in  a  thousand ;  but, 
as  it  appears  as  a  meshwork  of  long  filaments,  it  converts  the 
whole  into  a  general  jelly-like  mass,  which  has  a  still  more  solid 
appearance  if  it  is  formed  of  the  whole  blood,  so  that  the 
corpuscles  are  included  in  the  meshes. 

The  clotting  of  blood — that  is,  the  formation  of  the  in- 
soluble fibrin  from  the  soluble  fibrinogen — is  affected  by  various 
circumstances,  some  of  which  retard  or  entirely  prevent  it, 
while  others  accelerate  it.  A  study  of  these  has  thrown  some 
light  on  the  nature  of  the  changes  that  occur;  they  may  be 
summarized  as  follows  : — 

1.  A  certain  optimwn  temperature^  correspondifig  with  that  of 
the  body  or  a  little  over  it,  is  most  favourable  for  coagulation. 
As  the  temperature  sinks  below  this  point,  the  speed  of  coagu- 
lation diminishes  until  at  the  temperature  of  melting  ice  it 
becomes  infinitely  slow,  so  that  blood  may  be  kept  at  0°  C.  for 
days  without  undergoing  coagulation.  During  this  period,  the 
corpuscles,  which  are  heavier,  sink  to  the  bottom,  and  the  clear 
yellow  plasma  appears  above  them.  "WTien  they  have  com- 
pletely subsided,  the  moist  corpuscles  occupy  about  one-third 
of  the  volume.  If  this  iced  plasma  be  decanted  off  and  allowed 
to  gain  the  ordinary  atmospheric  temperature,  it  slowly  coagu- 
lates ;  if  it  be  heated  in  a  water  bath  to  the  temperature  of  the 
body,  it  coagulates  much  more  rapidly.  It  hence  contains  all 
the  necessary  factors  for  coagulation,  but  their  interaction  is 
prevented  by  the  low  temperature. 

2.  The  presence  of  an  excess  of  neutral  salts ^  such  as  sodium 
or  jnagnesium  sulphate,  delays,  and  in  sufficient  q^iantity  preve?ifs, 
the  coagulation  of  blood.  Hence  plasma  may  be  obtained  by 
drawing  off  the  blood  from  an  artery  into  a  sufficient  amount 
of  a  saturated  solution  of  such  a  neutral  salt.  To  prepare  such 
salted  plasma,  about  one-third  as  much  of  a  saturated  solution 
of  sodium  or  magnesium  sulphate  is  taken  as  there  is  blood 
expected,  and  the  blood  is  mixed,  by  stirring,  with  this  fluid 


126  Elementary  Physiology. 

as  it  flows  out  of  the  blood-vessel. ^  This  plasma  coagulates 
when  diluted,  and  still  more  rapidly  if  it  be  heated  and  have  a 
portion  of  blood-clot  from  another  source,  or  of  blood  serum,^ 
added  to  it.  It  is  of  service  in  investigating  the  problems  of 
coagulation,  because  the  fibrinogen  which  causes  coagulation 
may,  by  certain  means,  be  separated  from  it. 

3.  Blood  may  be  prevejited  from  coagulating  by  removing 
its  calciiLm  salts ^  or  converting  these  into  an  insoluble  form. 
This  may  be  done  by  mixing  the  blood  with  a  solution  of  a 
soluble  oxalate  (about  0*2  grammes  of  potassium  or  ammonium 
oxalate  to  each  100  cubic  centimetres  of  blood)  when  oxalate 
plasma  is  obtained.  This  clots  when  excess  of  a  soluble 
calcium  salt  is  added  to  it,  especially  on  warming. 

4.  Certain  substances,  such  as  "  Wittes^  peptone^''  ^  leech 
extract^  and  nmssel  extract,  if  previously  injected  into  a  vein 
of  a  living  animal,  prevent  the  clotting  of  its  blood  when  this 
is  drawn  soon  afterwards.  Plasma  prepared  by  this  method 
is  called  peptone  plasma. 

5.  Certain  other  substances  when  injected  into  the  veins, 
on  the  other  hand,  cause  coagulation  to  set  in  within  the  living 
blood-vessels  {intra-vasciilar  coagtUation).  These  substances 
can  be  extracted  from  the  testis  or  thymus,  or  other  glands 
rich  in  cell-nuclei;  they  were  named  tissiLe  fibrinogens  by 
their  first  discoverer  (Wooldridge),  but  are  probably  derived 
from  the  nuclei  of  the  cells  of  the  gland  extracted,  and  belong 
to  a  class  of  compounds  known  as  nucleo-albumins. 

6.  Contact,  with  foreign  bodies,  especially  if  these  have 
roughened  surfaces,  hastens  clotting,  and  remaining  in  contact 

'  The  preparation  of  the  plasma  in  this,  as  in  some  other  methods  given 
in  the  text,  may  be  hastened  by  separating  the  corpuscles  by  means  of  the 
centrifuge. 

^  Blood  serum  is  what  is  left  of  the  plasma  after  the  clot  has  separated  ; 
it  is  the  clear  fluid  which  separates  from  a  blood-clot  some  time  after  it  has 
formed. 

^  Blood-plasma,  like  the  other  fluids  of  the  body,  always  contains  a 
trace  of  soluble  calcium  salts.  In  the  preparation  of  "oxalate  plasma," 
these  are  not  removed  by  filtration,  but  only  thrown  out  of  solution  by  the 
addition  of  excess  of  a  soluble  oxalate, 

■*  A  mixture  of  albumoses  and  peptone,  obtained  as  a  result  of  the 
peptic  digestion  of  proteid  (see  p.  148) ;  the  anti-coagulative  action  is  due 
to  the  albumoses  present,  and  not  to  the  peptone. 


The  Blood.  127 

with  the  blood-vessels,  delays  it.  Blood  drawn  off  into  an 
oiled  vessel  is  much  longer  in  clotting  than  when  drawn  into  a 
dry  vessel ;  it  remains  still  longer  fluid  if  drawn  off  in  drops  into 
a  quantity  of  oil.  Again,  if  the  large  jugular  vein  of  the  horse 
be  ligatured  at  two  places  some  distance  apart,  so  as  to  be  full 
of  blood, ^  and  removed,  the  blood  will  remain  fluid  within  the 
vein  usually  for  some  days,  and  plasma  can  be  obtained  from 
it,-  which  soon  clots  when  drawn  oft' into  another  vessel.  The 
blood  does,  however,  occasionally  coagulate  even  within  the  ves- 
sels, and  is  usually  found  clotted  in  the  heart  soon  after  death. 

7.  Addition  of  old  blood-clot  to  a  sample  of  blood  or 
plasma  which  only  clots  slowly,  hastens  the  process. 

Fibrinogen^  that  proteid  of  the  plasma  which  causes  coagu- 
lation of  the  blood,  can  easily  be  obtained  from  any  of  the 
forms  of  plasma  of  which  the  mode  of  preparation  has  been 
indicated  above  by  adding  to  them  an  equal  volume  of  saturated 
sodium  chloride  solution,  that  is,  by  half  saturation  with  sodium 
chloride.  The  fibrinogen  appears  as  a  flocculent  precipitate, 
which  may  be  washed  with  some  half-saturated  sodium  chloride 
solution,  and  then  re-dissolved  by  adding  distilled  water,^  which 
forms  a  dilute  saline  with  the  adhering  sodium  chloride.  After 
being  reprecipitated,  washed,  and  redissolved  several  times, 
the  fibrinogen  is  finally  obtained  in  fairly  pure  solution.  It 
does  not  clot  spontaneously,  in  this  condition,  however  long  it 
be  kept  at  a  favourable  temperature,  but  only  after  two  things 
have  been  added  to  it.  One  of  these  is  a  soluble  calcium  salt," 
and  the  other  is  the  material  known  as  "  fibrin  ferment." 

Fibrin  fe7'menf  is  formed  after  the  blood  has  been  shed,  and 
is  believed  to  be  derived  from  the  nuclei  of  the  white  blood 
corpuscles  as  these  disintegrate.  Like  those  substances  which 
cause  intra- vascular  clotting  when  injected  into  the  veins  of  a 
living  animal,  it  belongs  to  the  class  of  proteids  called  nucleo- 
proteids,  which  are  found  in  cell  nuclei.  This  ferment,  or 
enzyme,  in  the  presence  of  a  soluble  calcium  salt,  converts  the 

^  To  attain  this  object,  the  ligature  nearer  the  heart  must  first  be  tied, 
then  the  other  one. 

-  This  experiment  was  first  performed  by  Hewson,  and  is  known  as  the 
Ihivg  test-Uibe  experiment. 

^  "Distilled  "  in  order  to  avoid  adding  calcium  salts. 


128  Elementary  Physiology. 

fibrinogen  of  the  plasma  into  an  insoluble  substance  called 
fibrin.  When  the  fibrin  includes  the  blood  corpuscles  in  its 
meshes,  a  blood-clot  is  the  result.  If  the  blood  be  whipped 
with  a  feather,  or  a  bundle  of  twigs  or  wires,  the  fibrin  of  the 
blood  separates  on  this,  and  may  be  washed  and  preserved. 
After  its  removal  the  blood  does  not  clot,  and  is  known  as 
whipped  blood}  After  blood  has  clotted,  a  clear  yellow  fluid, 
which  does  not  clot  again,  is  gradually  forced  out  from  the 
mass  of  the  clot ;  this  fluid  is  blood  senmi, 

.  Serum  differs  in  composition  from  plasma  only  in  that  it 
^contains  no  fibrinogen ;  for  this  has  all  been  converted  into 
fibrin,  and  remains  in  the  clot.  It  is  more  easily  obtained 
than  plasma,  and  since  it  differs  so  little  in  composition,  may 
be  used  to  study  the  properties  of  the  nutrient  fluid  in  which 
the  blood  corpuscles  float.  It  contains  about  ten  per  cent,  of 
proteid,  of  which  about  half  is  a  proteid  called  serum-glohdin^ 
which  is  precipitated  by  completely  saturating  the  serum  with 
magnesium  sulphate,  by  adding  crystals  of  that  salt,  or  else  by 
half  saturating  with  ammonium  sulphate  by  adding  an  equal 
volume,  to  the  serum,  of  the  saturated  solution  of  that  salt. 
The  remainder  of  the  proteid  is  a  mixture  of  substances  closely 
allied  to  one  another,  and  termed  serum-albumins ;  these  are 
not  precipitated  on  saturation  with  magnesium  sulphate,  or 
half  saturation  with  ammonium  sulphate,  but  are  precipitated 
on  complete  saturation  with  ammonium  sulphate. 

Another  method  of  separating  these  two  forms  of  proteid 
is  to  place  the  serum  in  a  dialyzing  tube,  made  of  parchment 
paper,  round  the  outside  of  which  a  current  of  water  is  made 
to  flow.  The  inorganic  salts  of  the  serum  pass  through  the 
parchment  paper,  but  the  proteids  cannot  pass,  and  so  remain 
within  the  dialyzer.  As  soon  as  the  salts  have  been  removed, 
the  serum-globulin  is  precipitated;  for  the  globulins  are  insoluble 
in  water  alone,  and  only  remain  soluble  in  blood  plasma  because 
of  the  inorganic  salts  which  it  also  has  in  solution.  The  serum- 
albumins  remain  in  solution,  and  can  be  separated  by  filtration. 
Afterwards,  the  globulin  precipitate  can  be  redissolved  in  dilute 
saline  solution. 

^  Whipped  blood  is  the  serum  with  the  corpuscles  suspended  in  it. 


TJie  Blood. 


129 


The  serum  also  contains  about  two  parts  per  thousand  of 
dextrose,  or  grape  sugar,  which  serves  as  carbohydrate  food  for 
the  tissues.  It  does  not  contain  fat  globules,  unless  it  be 
obtained  immediately  after  a  fatty  meal,  when  it  may  be  quite 
milky  from  suspended  fat. 

The  chief  inorganic  salts  are  sodium  chloride,  which  is 
present  in  largest  amount  (six  parts  per  thousand)  sodium 
carbonate  and  sodium  phosphate,  and  traces  of  calcium  salts. 
It  is  to  the  mixture  of  sodium  carbonate  and  phosphate  that 
serum  owes  its  alkaline  reaction.  Potassium  salts  are  present 
only  in  traces,  but  are  present  in  greater  amount  in  the 
corpuscles  and  in  the  cells  of  the  tissues. 

Besides  these  substances,  the  serum  contains  very  small 
amounts  of  other  organic  substances,  the  products  of  tissue 
activity,  which  are  kept  down  to  a  minimum  by  being  either 
excreted  by  the  kidneys  in  the  urine,  or  converted  into  other 
substances  as  the  blood  passes  through  the  liver. 


K 


CHAPTER   VIL 

DIET,    DIGESTION,    ABSORPTION,    AND   METABOIISM. 

The  supplies  of  nutrient  materials  which  the  blood  carries 
round  to  the  various  tissues  and  organs  of  the  body  are 
prepared  from  the  food  of  the  animal  by  a  process  called 
digestion.  While  undergoing  digestion,  the  food  slowly  passes 
along  a  tube  called  the  alimentary  canal,  which  is  really  an 
invagination  or  folding  in  of  the  outer  surface  of  the  body; 
straight  in  its  upper  and  convoluted  in  its  lower  part.  Hence, 
during  the  process  of  digestion,  the  food  is  not  really  within 
the  body,  but  only  within  a  hollow  tube  which  passes  through 
it,  and  it  is  only  after  being  digested,  and  having  passed 
through  the  walls  of  this  alimentary  canal  that  it  really  enters 
the  body.  The  indigestible  portion  which  cannot  be  absorbed, 
as  well  as  a  small  amount  of  material  excreted  into  the 
alimentary  canal  by  certain  glands  (chiefly  the  liver),  is  finally 
ejected  from  the  canal  at  its  lower  end. 

A  number  of  glands  pour  their  secretions  into  the  alimentary 
canal  at  various  points  along  its  length,  and  these  secretions 
contain  substances  which  act  on  the  various  constituents  of  the 
food,  and  form  from  these  soluble  products  which  are  easily 
absorbed  or  taken  up  by  the  cells  lining  the  walls  of  the 
alimentary  canal,  and  .after  undergoing  certain  modifications 
in  their  passage  through  these  cells  are  passed  onwards  to 
finally  reach  the  blood-stream.  This  process  of  absorption 
takes  place  chiefly  from  the  lower  part  of  the  alimentary  canal 
— that  is,  from  the  intestine.  It  may  be  well,  before  treating  of 
the  process  of  digestion,  to  briefly  summarize  the  steps  by  which 
the  animal's  food  is  prepared  for  it  by  other  natural  processes. 

The  sun  is  the  fundamental  source  of  all  that  energy  in  the 


Diet,  Digestion,  Absorption^  and  Metabolism.     131 

form  of  organic  life  which  we  see  exhibited  by  the  teeming 
myriads  of  plants  and  animals  inhabiting  our  planet.  Not 
only  is  this  true  in  the  sense  that  without  the  warmth  of  the 
sun's  rays  all  life  on  the  earth  would  be  impossible,  but  in  the 
narrower  sense  that  every  act  of  a  living  creature  requiring 
the  expenditure  of  energy  is  carried  out  by  energy  which  has 
been  stored  up  from  the  solar  rays.  The  solar  energy  is 
converted  first  into  chemical  energy  by  the  agency  of  living 
plants.  Every  green  plant  is  a  laboratory  in  which  the  energy 
of  the  sunlight  is  transmuted  into  the  energy  of  chemical 
substances,  which  can  afterwards,  it  may  be,  serve  in  the  form 
of  food  as  a  source  of  energy  for  the  supply  of  an  animal,  or 
after  elaboration  in  the  body  of  one  animal  may  supply  food 
{i.e.  chemical  energy)  to  another  animal. 

Plants  form  their  substance  from  inorganic  materials. 
They  build  up  from  these  simple  bodies  others  much  more 
complex  in  their  nature,  which  are  capable  in  passing  back 
again  to  the  simple  inorganic  forms  of  giving  out  a  supply  of 
energy  which  may  exhibit  itself  in  other  forms,  such  as  heat 
and  muscular  work. 

The  process  of  formation  of  the  more  complex  substances 
is  spoken  of  as  reduction  by  the  chemist.  In  it  energy  is 
required,  and  it  can  only  take  place  when  some  source  of 
energy  is  available  (sunlight  in  the  case  of  plant  life).  The 
opposite  process  in  which  the  simpler  substances  are  again 
formed  is  spoken  of  as  comhustioji  or  oxidation.^  because  usually 
oxygen  ^  is  used  up  in  the  process.  Here  energy  is  set  free, 
and  is  perceptible,  if  the  oxidation  takes  place  in  the  body  of 
an  animal,  in  the  heat  which  maintains  the  animal's  temperature 
above  its  surroundings,  and  in  the  muscular  movements  which 
are  continually  taking  place. 

In  plants,  then,  processes  of  reduction  go  on,  and  from 
very   simple   bodies    others    of  complex  chemical  nature  are 

^  A  gas  forming  about  one-fifth  of  the  atmosphere,  which  combines  with 
bodies  when  these  burn  (or  are  oxidized),  so  giving  rise  to  bodies  (oxides) 
with  a  less  store  of  chemical  energy  than  the  original  bodies.  The  chemical 
energ}'  so  dissipated  takes  the  form  of  heat,  light,  electricity,  sound, 
muscular  movement,  or  one  other  of  the  forms  of  energ}- ;  for  energy  can 
never  be  destroyed,  but  merely  transmuted  from  one  form  to  another. 


T32  Elementary  Physiology. 

formed  by  the  aid  of  energy  derived  from  the  solar  rays.  In 
animals  this  store  of  energy  is  taken  possession  of ;  the  animal, 
after  various  preliminary  processes,  absorbs  into  its  body  the 
complex  materials  forming  the  substances  of  the  plants,  and 
these  are  gradually  oxidized  (by  means  of  oxygen  taken  in  by 
the  lungs)  in  the  body  back  to  simpler  bodies.  In  the  process 
of  oxidation  the  solar  energy,  which  had  been  stored  up  by 
the  plant  as  chemical  energy,  is  again  set  free  in  the  form  of 
heat  and  of  muscular  work,  by  means  of  which  the  animal  is 
enabled  to  carry  on  its  existence.  It  must  not,  however,  be 
rashly  supposed  that  in  plants  the  life  processes  are  purely 
reductions  and  accumulations  of  energy,  and  in  animals  the 
reverse.  It  is  only  true  that  the  total  effect  in  a  plant  is  a 
preponderance  of  reduction  and  an  accumulation  of  chemical 
energy,  and  in  an  animal  the  opposite  is  true ;  but  at  the  same 
time  the  reversed  processes,  only  in  less  degree,  go  on  in  the 
two  kingdoms  of  life. 

To  enter  a  little  more  into  detail,  plants  take  up  carbon 
dioxide  from  the  atmosphere,  which  contains  that  gas  to  the 
extent  of  three  to  four  parts  per  ten  thousand,  and  by  the  aid  of 
sunlight  assimilate  the  carbon  of  the  carbon  dioxide,  and  set  free 
its  oxygen.  From  the  supply  of  carbon  so  obtained,  and  the 
water  of  their  tissues,  plants  build  up  more  and  more  complex 
organic  substances  ;  also  with  the  addition  of  nitrogen  salts 
obtained  from  the  soil  even  more  complex  organic  substances 
containing  nitrogen  are  formed.  These  nitrogenous  organic 
bodies  form  an  indispensable  item  of  animal  food,  and  are 
known  as  vegetable  proteids.  Although  the  organic  bodies 
found  in  the  dried  substance  of  plants  are  very  numerous,  by 
far  the  greater  weight  of  their  organic  material  belongs  to  one 
of  three  great  classes  of  organic  bodies,  which  are  termed 
carhohydrates.,  fats^  and  proteids. 

In  carbohydrates  the  elements  present  are  carbon^  hydrogen^ 
and  oxygen.  Of  these  elements,  hydrogen  and  oxygen  are 
present  in  the  exact  proportions  required  to  form  water,^  and 

^  It  must  not  be  supposed,  however,  that  water  is  present  in  the  carbo- 
hydrate molecule.  The  hydrogen  and  oxygen  happen  to  be  present  in  the 
same  proportion  as  they  are  present  in  water  ;  but  they  are  combined  with 
the  carbon  in  a  comj^lex  manner,  and  not  so  as  to  form  water. 


Diet,  Digestion,  Absorption,  and  Metabolism.     133 

the  ratio  of  the  carbon  to  the  hydrogen  and  oxygen  is  variable 
in  the  different  groups  of  the  class.  AVhen  carbohydrates  are 
burnt,  since  there  is  sufficient  oxygen  in  the  molecule  to  com- 
bine with  all  the  hydrogen,  oxygen  is  required  only  for  the 
carbon,  and  as  much  carbon  dioxide  is  formed  by  volume  as 
there  is  oxygen  used  up.  Hence,  if  an  animal  could  be  fed 
purely  on  carbohydrates,  it  would  give  out  as  much  carbon 
dioxide  by  its  lungs  as  it  took  in  oxygen.  In  the  case  of  the 
other  two  classes  of  food-stufifs  (fats  and  proteids),  there  is  not 
enough  oxygen  in  the  molecule  to  combine  completely  with  all 
the  hydrogen  present  to  form  water  during  combustion.  Hence 
some  of  the  oxygen  taken  in  by  the  lungs  combines  in  the 
tissues  with  this  excess  of  hydrogen  to  form  water,  and  only 
the  remainder  which  combines  with  the  carbon  reappears,  in 
the  carbon  dioxide  exhaled  by  the  lungs,  in  gaseous  form. 
When  fats  and  proteids  are  present  in  the  food,  therefore,  as 
they  always  are,  there  is  less  carbon  dioxide  given  out  by 
volume  through  the  lungs  than  there  is  oxygen  taken  in.  The 
ratio  of  carbon  dioxide  given  out  to  oxygen  taken  in  is  spoken 
of  as  the,  respiratory  qtwtient ;  it  is  increased  by  carbohydrate, 
diminished  by  proteid,  and  still  more  by  fatty  food.^ 

The  chief  groups  of  carbohydrates  slxq  glucoses,  saccharoses, 
and  amy  loses,  or  starches  P- 

The  glucoses  (CgHiaOe)  are  the  simplest  members ;  examples 
of  them  are  dextrose,  or  grape  sugar,  and  IcEvulose,  which  is 
formed  in  the  inversion  of  cane  sugar. 

The  saccharoses  (C12H22O11)  are  somewhat  more  complex 
in  constitution ;  when  treated  wdth  dilute  mineral  acid  they  are 
decomposed  into  glucoses  (inversion).  Examples  are  cane 
sugar,  maltose,  and  lactose,  or  viilk  sugar. 

The  starches  [(CeHioOj),,]  are  much  more  complex  than 
either  the  glucoses  or  saccharoses,  into  which  they  become 
converted  when  they  are  either  boiled  with  dilute  mineral  acids, 
or  are  treated  with  certain  ferments  contained  in  the  digestive 
juices.     Examples  of  starches  are  the  ordinary  potato  and  rice 

'  It  is  obvious  from  the  above  that  the  respiratory  quotient  never  can 
exceed  unity. 

2  A  more  recent  terminology  is  mono-saccharides,  di-saccharides,  and 
poly-saccharides. 


134  Elementary  Physiology. 

starch  of  commerce.  The  only  starch  occurring  in  the  animal 
body  is  a  substance  called  glycogen^  or  atiiinal  starchy  which  is 
found  in  the  liver,  and  to  a  less  extent  in  the  muscles.  Glycogen 
accumulates  temporarily  in  the  liver  after  carbohydrate  food, 
being  gradually  converted  into  sugar,  and  used  up  in  the  tissues 
afterwards. 

The  fats  contain  the  same  three  elements  as  the  carbo- 
hydrates, but,  as  stated  above,  in  quite  different  proportions. 
Chemically,  the  fats  are  glycerides — that  is,  they  are  formed  by 
a  combination  of  glycerine  with  a  fatty  acid  or  acids.  The  fatty 
acid  present  has  a  large  number  of  carbon  and  hydrogen  atoms 
in  its  molecule,  and  is  hence  a  very  weak  acid.  To  this  weak 
fatty  acid  the  glycerine  behaves  as  a  base,  so  that  the  fats  are 
neutral  bodies.  There  are  three  chief  fats  present  in  the  animal 
body,  viz.  olein,  palmitin,  and  stearin.  These  three  bodies  have 
different  melting-points,  and,  according  to  the  proportion  in 
which  they  are  mixed,  give  rise  to  the  different  physical  and 
other  characters  which  distinguish  the  fat  of  different  species 
of  animals. 

The  proteids  are,  in  their  chemical  composition,  the  most 
complex  class  of  bodies  found  in  the  animal  body.  They 
consist  of  carbon,  hydrogen,  oxygen,  nitrogen,  sulphur,  and 
sometimes  phosphorus,  united  in  proportions  which  vary  some- 
what for  different  members  of  the  class.  Their  chemical  con- 
stitution is  at  present  unknown,  but  it  is  certain,  from  various 
properties  which  they  possess — such  as  inability  to  diffuse 
through  membranes,  small  percentage  of  sulphur  or  phosphorus, 
and  large  number  of  decomposition  products — that  they  possess 
high  molecular  complexity.  Proteids  form  the  chemical  basis 
of  protoplasm,  and  are  hence  present  in  every  living  cell  of 
the  body.  For  the  same  reason  they  are  an  indispensable 
form  of  food  for  all  animals,  since  they  are  necessary  for  the 
repair  of  protoplasmic  waste.  Thus,  while  an  animal  can  be 
kept  alive  when  fed  on  proteid  alone,  and  denied  all  fat  or 
carbohydrate,  its  life  cannot  be  sustained  on  a  diet  of  pure  fat 
or  pure  carbohydrate,  or  a  mixture  of  these  two  food-stuffs. 

Proteid  might,  therefore,  be  designated  as  an  essential  food, 
and  carbohydrate  and  fat  as  accessory  foods,  but  that  this  would 


Diet,  Digestion,  Absorption,  and  Metabolism.     135 

minimize  too  much  the  importance  of  fats  and  carbohydrates 
as  food-stuffs.  For,  although  an  animal  can  be  kept  alive  on 
proteid  alone,  this  forms  a  very  inefficient  and  imperfect  diet.  A 
part  of  the  proteid  so  eaten  goes  to  do  ^vork  which  can  be 
much  better  done  by  carbohydrate  or  fat.  Just  as  the  chief 
purpose  of  the  proteid  of  the  food  is  to  renovate  the  cell 
protoplasm,  to  repair  the  waste  of  cell  substance,  the  chief  use 
of  carbohydrate  and  fat  is  to  supply  chemical  energy  for  cell 
activity.  When,  for  example,  a  muscle  contracts,  it  does  so 
mainly  at  the  expense  of  chemical  energy  supplied  by  the  com- 
bustion in  the  muscle  of  carbohydrate  material.  In  the  absence 
of  carbohydrate,  proteid  may  be  used  as  a  source  of  energy, 
but  it  is  less  effectual,  and  there  is  a  waste  in  its  application. 
When  the  muscles  are  set  hard  at  work  contracting,  as  when 
prolonged  exercise  is  taken,  if  a  sufficient  supply  of  carbo- 
hydrate and  fatty  food  be  given,  there  is  found  to  be  little 
increase  in  the  amount  of  proteid  used  up  in  the  body,  but  a 
considerable  increase  in  the  amount  of  fat  and  carbohydrate 
used.^ 

The  only  difference  in  importance,  then,  between  proteid 
on  the  one  hand,  and  carbohydrates  and  fats  on  the  other,  is 
that  proteids  alone  can  repair  protoplasmic  waste. 

A  proper  diet  hence  consists  of  a  judicious  mixture  of  the 
three  great  classes  of  foodstuffs,  and  this  is  the  diet  which 
we  naturally  select  in  the  mixture  of  foods  which  we  eat  from 
day  to  day. 

The  digestive  apparatus  of  man  is  intermediate  in  type 
between  that  of  herbivora  and  that  of  carnivora,  as  is  shown 
by  the  form  of  the  teeth  and  the  length  of  the  alimentary 
canal.  Such  a  natural  condition  of  affairs  indicates  a  mixed 
diet  of  flesh  and  vegetables  as  the  best  suited  for  our 
consumption. 

^  This  is  shown  by  determining  the  amount  of  urea  (the  end  product 
of  proteid  change  in  the  body)  excreted  while  the  amount  of  muscular  work 
is  varied.  The  excretion  is  then  found  to  be  little  changed,  the  increase 
being  quite  insufficient  to  account  for  the  work  done,  and  merely  repre- 
senting the  increased  wear  and  tear  on  the  protoplasm.  On  the  other 
hand,  the  amount  of  carbon  dioxide  given  off  from  the  lungs  is  found  to  be 
largely  increased,  thus  pointing  to  an  increased  consumption  of  carbo- 
hvdrate  or  fat. 


136  Elementary  Physiology. 

The  objections  urged  by  vegetarians  on  ethical  grounds 
against  the  slaughter  of  animals  for  food  are  utterly  opposed 
to  natural  law.     For  there  are  whole  classes  of  animals  which 
can  only  exist    on  animal   food,  and   throughout   nearly  the 
whole  of  the  animal  world  one  species  preys  upon  another; 
the   stronger  attacking,   killing,   and    devouring   the   weaker. 
Indeed,  animal  life  cannot  be  maintained  except  by  preying 
on  other  forms  of  life ;  the  animal  organism,  of  whatever  type  it 
he,  cannot  prepare  its  food  from  inorganic  sources,  but  must 
ultimately,  directly  or  indirectly,  sustain  itself  on  plants,  which 
are  also  living,  and  must  die  to  supply  its  food.     The  diet 
used  by  an  animal  varies  with  many  circumstances.     It  varies 
first  of  all  with  the  class  of  animal.     A  herbivorous  animal 
has  a  capacious  stomach  and  intestine,  and  fares  best  with 
coarse,  bulky  food,  such  as  vegetables,  grass,  and   hay.     A 
carnivorous  animal  has  a  much  less  capacious  alimentary  canal, 
the  intestine  is  very  short,  and  hence  some  food  is  necessary 
which  contains  a  large  amount  of  nutriment  in  an  easily  avail- 
able  form,   and   in  a  small   bulk,  so  that  it  can  be  readily 
digested  and  absorbed.     Such  an  animal,  therefore,  is  best  fed 
on  flesh  meat.     The  food  varies  again  with  the  climate  and 
time  of  year.     Fats  are  the  form  of  foodstuff,  which  produce 
in  combustion  the  greatest  amount  of  heat  from  a  given  weight, 
for  they  evolve  nearly  twice  as  much  heat  as  an  equal  weight 
of  either  proteid  or  carbohydrate.^     Hence  in  cold  climates  or 
in  winter  time  much  more  fat  is  naturally  taken  in  the  food 
than  in  warm  climates  or  in  the  summer. 

The  amount  of  food  required  by  an  individual  varies  with 
the  extent  of  his  exercise  or  labour.  Hard  work  requires  a 
liberal  allowance  of  food,  and  with  a  sedentary  occupation 
the  amount  of  food  must  be  diminished,  or  troubles  of  diges- 
tion and  nutrition  arise.  About  a  quarter  of  the  food  daily 
taken  is  solid,  and  the  remainder  consists  of  water.  Various 
normal  diets  have  been  given  by  different  observers,  but  the 
conditions  are  so  variable  as  to  make  these  of  little  value. 
The  two  most  often  quoted  are  the  following,  which  are  given 

^  The  heats  of  combustion  of  proteid  and  carbohydrate  are  very  nearly 
equal. 


Diet,  Digestion,  Absorptio7i,  and  Metabolism.     137 

in  round  numbers,  in  weights  of  dry  solids,  for  an  average  man 
weighing  70  kilograms,  or  about  150  pounds: — Proteid,  120 
grammes ;  ^  fat,  50-100  grammes ;  carbohydrate,  350-500 
grammes  (Voit)  : — Proteid,  100  grammes  ;  fat,  100  grammes; 
carbohydrate,  240  grammes  (Ranke). 

Besides  the  three  classes  of  organic  foodstuffs  mentioned 
above,  it  is  necessary  to  hfe  that  a  supply  of  inorganic  salts 
should  be  taken  in  with  the  food,  for  each  day  a  considerable 
amount  of  these  is  excreted  in  the  urine.  These  are  in  part 
contained  in  the  food  itself,  and  in  part  dissolved  in  the  water 
which  is  drunk  with  it,  while  in  addition  we  daily  consume  a 
certain  amount  of  common  salt  with  our  food. 

Movements  of  the  Alimentary  Canal. 

The  alimentary  canal  is  lined,  almost  throughout  its  entire 
length,  by  muscular  fibres,  which  are  arranged  in  two  coats.  In 
the  inner  coat  (that  next  the  lumen  of  the  tube)  the  fibres  are 
disposed  circularly  round  the  tube  to  form  the  circular  coat ; 
while  in  the  outer  coat  the  fibres  are  arranged  parallel  to  the 
length  of  the  tube,  and  constitute  the  longitudinal  coat.  It  is 
by  the  contraction  of  these  muscle  fibres  in  turn  that  a  wave 
of  contraction  is  caused  to  pass  along  the  tube,  so  gradually 
shifting  onward  the  food  which  is  undergoing  digestion.  The 
muscles  of  the  cheek  and  pharynx,  and  of  the  upper  part  of 
the  oesophagus,  are  striped ;  but  the  muscle  fibres  lining  the 
remainder  of  the  alimentary  track  are  involuntary,  except  at 
the  anus,  where  the  fibres  of  the  external  sphincter  are  striped. 

Mastication,  or  chewing,  consists  in  the  comminution  of  the 
food  by  grinding  it  between  the  teeth,  under  which  it  is  repeatedly 
placed  by  the  action  of  the  muscles  of  the  cheeks  and  tongue. 
When  the  food  has  been  sufficiently  broken  up  by  the  teeth  it 
is  swallowed,  and  passes  down  the  oesophagus  into  the  stomach. 
In  the  process  of  deghttition^  or  swallowing,  the  food,  which  has 
been  rolled  into  a  rounded  mass  or  bolus  by  the  action  of  the 
cheek  and  tongue  muscles,  is  slid  back  over  the  tongue,  and 
carried  back  on  the  base  of  the  tongue  into  the  pharynx.     So 

'  There  are  about  440  grammes  in  one  pound  avoirdupois. 


138  Elementary  Physiology. 

far  the  process  is  voluntary,  but  the  remainder  is  involuntary. 
While  in  the  pharynx,  and  before  it  enters  the  oesophagus,  the 
food  is  in  the  way  of  the  air  passing  to  and  fro  between  the  nose 
or  mouth  and  the  trachea,  and  hence  during  this  second  stage 
of  its  journey  the  muscular  movements  are  very  rapid,  and  respi- 
ration is  suspended  while  they  take  place.  The  soft  palate  at 
the  back  of  the  roof  of  the  mouth  is  raised,  shutting  off  the 
nasal  passage  from  the  pharynx,  and  making  a  wide  passage 
for  the  bolus  of  food ;  the  trachea  and  its  upper  opening,  the 
glottis,  are  pulled  up  in  front  beneath  the  base  of  the  tongue, 
so  as  to  prevent  any  possibility  of  food  entering  the  trachea ; 
the  bolus  of  food  is  grasped  in  turn  by  each  of  three  pairs  of 
muscles,  the  constrictors  of  the  pharynx,  which  contract  upon 
it,  andy^;r^  it  downwards  into  the  upper  end  of  the  oesophagus. 
The  third  act  in  the  process  of  deglutition  consists  in  the 
passage  of  the  food  along  the  oesophagus  to  the  stomach ;  this, 
in  man,  is  assisted  by  the  action  of  gravity,  but  it  is  easy  to 
swallow  upwards,  and  many  animals  habitually  do  so.  The 
passage  is  caused  by  a  peristaltic  wave,  which  consists  of  an 
annular  constriction  passing  along  the  oesophagus  from  its 
upper  to  its  lower  end,  and  forcing  the  food  in  front  of  it. 

The  peristalsis  of  the  oesophagus  differs  from  that  of  the  intestine  to  be 
presently  mentioned  in  that  it  takes  place  under  the  direct  action  of  nerve 
impulse.  If  the  oesophagus  be  ligatured  or  cut  across,  the  wave  passes  the 
point,  and  is  continued  on  uninterrupted  on  the  other  side.  On  the  other 
hand,  if  the  nerves  to  the  oesophagus  be  cut,  the  peristalsis  does  not  take 
place.  In  the  case  of  the  intestine  the  reverse  is  the  case  in  both  instances  ; 
section  of  nerves  does  not  stop  the  peristalsis,  but  section  of  the  muscle 
fibres  does.  Hence  it  is  probable  that  the  peristalsis  of  the  oesophagus  is 
a  succession  of  reflex  discharges  of  nerve  impulses  along  the  tube,  while 
that  of  the  intestine  is  a  muscular  contraction  propagated  from  muscle  fibre 
to  muscle  fibre. 

In  the  stomach  there  is  an  oblique  layer  of  muscular  fibres 
interposed  between  the  inner  circular  and  the  outer  longi- 
tudinal coats.  By  slow  peristaltic  contractions  of  these  various 
layers,  which  become  more  energetic  one  or  two  hours  after 
a  meal,  the  food  is  churned  about  in  the  stomach,  and  at  a 
later  period,  by  more  forcible  contractions,  it  is  forced  out  at 


Diet,  Digestion,  Absorptio7t,  and  Metabolism.     139 

intervals  through  the  pylorus  ^  into  the  duodenum,  or   upper 
part  of  the  small  intestine. 

Slow  peristaltic  waves  pass  along  the  intestine  in  the  form 
of  annular  constrictions,  which  move  the  food  slowly  down  the 
intestine.  These  peristaltic  waves  may  be  well  seen  when  the 
abdomen  of  a  freshly  killed  animal  is  opened,  for  the  cold  of 
exposure  increases  them.  They  may  be  artificially  started 
at  any  point  by  touching  with  the  point  of  a  knife.  After  a 
considerable  pause,  for  the  latent  period  is  very  long,  an  annular 
constriction  begins  at  the  point  touched,  and  usually  a  con- 
stricted wave,  which  travels  very  slowly,  passes  from  this  point 
both  up  and  down  the  intestine.  These  intestinal  movements 
are  stimulated  and  increased  by  the  presence  of  food  in  the 
intestine,  and  are  diminished,  and  gradually  subside  when  the 
intestine  has  been  empty  of  food  for  some  time. 

The  Digestive  Glands. 

The  food  while  passing  along  the  alimentary  canal  is  acted 
upon  by  secretions,  which  are  poured  in  at  various  points  along 
the  length  of  the  canal.  In  some  instances  these  secretions 
are  poured  in  at  the  openings  of  comparatively  large  ducts, 
which  carry  the  secretion  that  has  been  collected  from  large 
glands  lying  at  some  distance  from  the  intestine.  In  other 
cases  the  glands  are  minute,  and  lie  in  great  numbers  in  the 
inner  or  mucous  coat  of  the  canal,  and  their  secretion  is  poured 
out  by  minute  ducts  upon  the  mucous  membrane  ^  which  forms 
the  inner  surface.  Examples  of  the  former  type  of  gland  are  the 
salivary  glands,  the  pancreas,  and  the  liver ;  and  of  the  latter 
there  are  the  gastric  glands  lying  in  the  mucous  coat  of  the 
stomach,  and  the  glands  of  Lieberkiihn,  which  are  small  tube- 
like glands  embedded  in  the  mucous  coat  of  the  intestine. 

*  The  stomach  is  closed  at  its  two  openings  (the  cardiac  orifice  and 
pyloric  orifice)  by  muscular  rings  called  sphincters  ;  that  separating  it  from 
the  oesophagus  is  termed  the  cardiac  sphincter,  and  that  separating  it  from 
the  duodenum  the  pyloric  sphincter.  These  rings  are  only  relaxed  in  order 
to  allow  food  to  pass  in  or  out  of  the  stomach,  and  the  food  does  not  pass 
out  continuously,  but  only  at  intervals  when  the  pylorus  is  opened. 

^  The  term  mucous  viembrauc  is  applied  to  a  surface  moistened  by  mucus 
which  is  secreted  by  some  of  the  cells  forming  the  surface.  Such  a  mucous 
membrane  lines  the  alimentary  tract,  as  well  as  the  respiratory  and  nasal 
passages,  which  may  be  regarded  as  prolongations  or  diverticula  of  it. 


I40 


Elementary  Physiology. 


Besides  these  glands  there  are  single  cells  in  large  numbers 
to  be  found  in  the  inner  lining  layer  of  the  intestine/  which, 
from  their  shape,  are  termed  goblet  cells.  These  goblet  cells 
secrete  mucus,  which  moistens  the  surface  of  the  intestine,  and 
may  be  regarded  as  the  simplest  type  of  gland  to  be  found  in 
the  body.  In  the  arrangement  of  their  cells,  the  salivary  glands 
and  the  pancreas  are  what  is  known  as  racemose  glands.  The 
cells  which  furnish  the  secretion  are  aggregated  in  little  lobules 


"^^" 


Fig.  69.— Section  of  the  submaxillary  gland  of  the  dog,  showing  the  commencement  of 
a  duct  in  the  alveoli.     (Magnified  425  diameters.) 

a,  one  of  the  alveoli,  several  of  which  are  in  the  section  shown  grouped  around  the  com- 
mencement of  the  duct  it' ;  a' ,  an  alveolus,  not  opened  by  the  section ;  b,  basement- 
membrane  in  section  ;  c,  interstitial  connective  tissue  of  the  gland ;  d,  section  of  a 
duct  which  has  passed  away  from  the  alveoli,  and  is  now  lined  with  characteristically 
striated  columnar  cells  ;  s,  semilunar  group  of  darkly  stained  cells  at  the  periphery 
of  an  alveolus. 

or  bunches,  which  are  composed  of  smaller  groups  of  cells 
called  acini  or  alveoli.  In  each  acinus  the  cells  are  arranged 
round  a  central  minute  duct,  or  hcmen^  which  serves  to  carry 
off  the  small  quantity  of  secretion  furnished  by  the  acinus. 
The  minute  ducts  leading  from  the  acini  unite  with  one  another 
to  form  larger  ducts,  which  again  unite,  until  finally  there  is 

1  Such  cells  are  also  found  amongst  the  ciliated  cells  of  the  trachea  and 
bronchi,  and  in  other  similar  situations. 


Diet,  Digestion,  Absorption,  and  Metabolism.      141 

formed  one  chief  duct,  which  carries  the  entire  secretion  of 
the  whole  gland.  The  whole  structure  is  thus  somewhat  like 
a  tree  branch  or  a  bunch  of  grapes  in  its  arrangement,  and 
it  is  for  this  reason  that  such  glands  are  called  racemose. 

An  idea  of  the  arrangement  of  the  cells  in  such  a  gland 
may  be  gathered  from  the  accompanying  drawings,  which  show 
the  appearance  presented  by  thin  sections  of  the  submaxillary 


Fig.  70.— Section  of  the  pancreas  of  the  dog.     (Klein.) 
d,  termination  of  a  duct  in  the  tubular  alveolus,  a. 

salivary  gland  and  of  the  pancreas  of  the  dog.  In  the 
salivary  glands  the  ducts  are  more  numerous  than  in  the 
pancreas.  The  alveoli,  or  acini,  are  also  much  shorter  and 
more  rounded  in  the  salivary  glands  than  in  the  pancreas, 
where  they  form  long  columns  of  cells.  The  bile  duct,  which 
carries  the  bile  from  the  liver  to  the  duodenum,  arises  in  a 
similar  branching  fashion  from  the  lobules  of  the  liver.  But 
the  secretion  of  bile  is  not  the  main  function  of  the  liver,  as 
that  of  the  saliva  is  of  the  salivary  glands  ;  it  has  other  impor- 
tant work,  which  will  be  indicated  later. 

There  are  three  pairs  of  salivary  glands,  known  as  the 
parotid,  submaxillary,  and  stiblingiial  glands  respectively,  and 
besides  these  there  are  a  large  number  of  much  smaller  glands 
lying  beneath  the  mucous  membrane  of  the  mouth,  and  opening 
upon  it  by  minute  ducts. 


142 


Elementary  Physiology. 


^  The  parotid  gland  (see  Fig.  71)  is  the  largest  of  the  three 
salivary  glands  and  weighs  nearly  one  ounce  (20  to  30 
grammes).  It  lies  in  front  of  and  below  the  ear,  and  extends 
deeply   into  the  cleft  behind   the    ramus   of  the   lower  jaw. 


Fig.  71. — Sketch  of  a  superficial  dissection  of  the  face,  showing  the  position  of  the 
parotid  and  submaxillary  glands.     (Allen  Thomson.)     | 

/,  parotid  gland;/',  socia  parotidis  ;  d,  the  duct  of  Stensen  before  it  perforates  the 
buccinator  muscle  ;  a,  transverse  facial  artery  ;  n,  n,  branches  of  the  facial  nerve 
emerging  from  below  the  gland  ;  /,  the  facial  artery  passing  out  of  a  groove  in  the 
submaxillary  gland  and  ascending  on  the  face;  S7n,  superficial  portion  of  the 
submaxillary  gland. 

Its  duct  (Stensen' s  duct)  leaves  the  gland  at  its  anterior  border, 
and  runs  forward  in  the  cheek  external  to  the  masseter  muscle, 
round  the  anterior  border  of  which  it  turns  and  passes  inward 
to  open  into  the  mouth  on  the  inner  surface  of  the  cheek 
opposite  to  the  second  molar  tooth  of  the  upper  jaw,  where 
there  is  a  small  papilla. 

The  submaxillary  gland  is  next  in  size,  weighing  about  a 


Diet,  Digestion,  Absorption,  and  Metabolism.     143 

quarter  of  an  ounce  (8  to  10  grammes).  It  is  ovoidal  in  form, 
and  is  situated  below  and  to  the  inner  side  of  the  base  of  the 
lower  jaw.  The  duct  (Wharton's  duct)  leaves  the  gland 
posteriorly,  and  then  turning  forward  and  inward  beneath  the 
subhngual  gland  (see  Fig.  72),  it  runs  forward  and  opens  into 
the  mouth,  close  to  its  fellow  of  the  opposite  side,  at  the 
frcBmim  lijigiirce^  that  band  which  binds  down  the  tongue  in  front. 
The  sublingual  gldiud  (see  Fig.  72)  is  much  smaller  than  the 
other  two.     It  lies  in  the  floor  of  the  mouth,  covered  only  by 


Fig.  72. — View  of  the  right  submaxillary  and  sublingual  glands  from  the  inside.     (Allen 

Thomson.) 

Part  of  the  right  side  of  the  jaw,  divided  from  the  left  at  the  symphysis,  remains  ;  the 
tongue  and  its  muscles  have  been  removed  ;  and  the  mucous  membrane  of  the  right 
side  has  been  dissected  off  and  hooked  upwards  so  as  to  expose  the  sublingual 
glands  ;  j  w,  the  larger  superficial  part  of  the  submaxillar^'  gland  ;  /,  the  facial 
artery  passing  through  it ;  j-  m',  deep  portion  prolonged  on  the  inner  side  of  the  m3-lo- 
hyoid  muscle  in  h ;  s  I,  is  placed  below  the  anterior  large  part  of  the  sublingual 
gland,  with  the  duct  of  Bartholin  partly  shown  ;  5  /',  placed  above  the  hinder  small 
end  of  the  gland,,  indicates  one  or  two  of  the  ducts  perforating  the  mucous  mem- 
brane ;  d,  the  papilla,  at  which  the  duct  of  Wharton  opens  in  front  behind  the  incisor 
teeth  ;  a! ,  the  commencement  of  the  duct ;  h,  the  hyoid  bone  ;  n,  the  lingual  nerve. 

mucous  membrane,  between  the  tongue  and  the  gums  of  the 
lower  jaw,  where  it  forms  a  slight  swelling.  The  sublingual 
gland  has  several  ducts  (ducts  of  Rivinus),  which  either  open 
separately  into  the  mouth  or  join  Wharton's  duct.  One  of  these 
is  often  larger  than  the  others  and  is  termed  the  duct  of 
Bartholin ;  it  usually  opens  into  the  submaxillary  duct. 

The  saliva  is  the  mixed  secretion  of  these  various  glands. 
It  is  a  thick  stringy  or  mucous  fluid,  which  serves  the  double 
purpose  of  moistening  the  mouth  and  the  food,  and  of  coating 
each  bolus  over  with  a  slippery  envelope  which  facilitates  its 
passage  to  the  stomach.  Its  stringiness  is  due  to  the  mucin 
which   it  contains  ;  this  may  be  precipitated  from  it  by   the 


144  Elementary  Physiology. 

addition  of  a  few  drops  of  acetic  acid.^  Besides  this  physical 
action  on  the  food,  saliva  has  in  herbivora  and  in  man  2  a 
chemical  action  upon  starch,  which  it  converts  into  a  sugar 
{maltose)  and  a  mixture  of  dextrins.^  This  conversion  is  brought 
about  by  traces  of  a  substance  <Z2^&^  ptyalin  which  it  contains. 
Ptyalin  is  the  first  example  which  we  have  met  of  those 
substances  called  unorganized  ferments,  or  enzymes^  to  which  the 
chemical  changes  in  the  food  brought  about  by  the  digestive 
juices  are  due.  These  peculiar  substances  have  not  yet  been 
isolated  by  the  physiological  chemist  in  a  pure  condition,  so  that 
their  chemical  nature  is  unknown.  They  exist  only  in  traces 
in  the  several  digestive  fluids  in  which  they  occur,  but  are  so 
powerful  in  their  action  that  this  is  no  disadvantage.  The  follow- 
ing are  the  chief  general  characteristics  of  their  mode  of  action  : — 

1.  They  all  act  best  at  a  certain  temperature  which  is  known 
as  the  optimum  temperature  ;  as  the  temperature  falls  below 
this  their  action  becomes  less  energetic  and  finally  ceases.  If 
the  fluid,  however,  be  warmed  again  they  become  active  once 
more ;  as  the  temperature  rises  above  the  optimum  point  the 
action  also  slackens,  and  at  a  certain  temperature  (60°  to  70°  C.) 
the  enzyme  becomes  permanently  destroyed  and  does  not 
work  again  when  the  temperature  is  lowered. 

2.  They  act  best  with  a  given  reaction  of  the  fluid  and  a 
definite  degree  of  acidity  or  alkalinity.  Most  of  them  act  in 
an  alkaline  medium,  hut  pepsi?i,  an  enzyme  of  the  gastric  juice, 
acts  only  in  an  acid  medium,  and  is  rapidly  destroyed  by 
alkalies ;  those  which  act  in  an  alkaline  medium  are,  on  the 
other  hand,  destroyed  by  acids. 

3.  The  action  of  all  enzymes  is  catalytic — that  is  to  say,  they 
are  not  changed  by  the  reaction  which  they  induce,  and  an 
indefinitely  small  amount  of  enzyme  will  change  an  indefinitely 
large  amount  of  substance,  except  in  so  far  as  it  is  lost  by 
dilution.  The  action  is  probably,  in  most  cases,  one  of  hydrolysis, 
or  the  taking  up  of  the  elements  of  water. 

^  It  remains  dissolved  in  the  saliva  because  of  the  alkaline  reaction  of 
that  fluid. 

2  This  action  is  wanting  in  typical  carnivora. 

2  Dextrins  are  carbohydrates  intermediate  between  saccharoses  and 
starches  in  their  properties. 


Diet.  Digestion,  Absorption,  and  Metabolism.     145 


4.  The  action  of  each  enzyme  is  specific — that  is,  each 
particular  enzyme  acts  only  on  one  particular  class  of  foodstuff, 
and  not  upon  all  three.  The  enzymes  are  classified  according 
to  the  class  of  food-stuff  they  act  upon,  and  the  manner  of  their 
action  upon  it.  Amylolyfic  enzymes  act  upon  starches,  and 
convert  them  into  maltose  and  dextrins.  Iiiverfing  enzymes 
act  upon  the  compound  sugars,  and  convert  them  into  simple 
sugars.  Proteolytic  enzymes  act  upon  proteids,  and  convert 
them  into  albumoses  and  peptones.  Steatolytic  enzymes  act  on 
fats,  and  convert  them  into  fatty  acids  and  glycerine.  Besides 
these  there  are  certain  enzymes  known  which  cause  coagula- 
tion of  fluids,  such  as  re/inin,  which  causes  milk  to  clot,  and 
filmn-fermenf,  which  produces  blood  coagulation. 

The  following  table  gives  an  enumeration  of  the  chief 
enzymes  of  digestion,  the  fluids  in  which  they  are  found,  the 
reaction  with  which  they  work,  and  a  summary  of  their  action  : — 


Name. 

Digestive 

fluid  in  which 
found. 

Reaction 
of  fluid. 

Action  on  food-stuff. 

I.  Amy lo lytic  enzymes: 

[a)  Ftyali7i 

Saliva 

Alkaline 

Converts  starch  into  malt- 
ose and  dextrins. 

{b)  A  my  lop  sin 

Pancreatic 
juice 

Do. 

Do. 

2.  Inverting  enzymes  : 

[a)  Invertin 

Intestinal 

Do. 

Converts  compound  sugars 

fluid  1 

such  as  cane  sugar  and 
maltose  into  simple 
sugars  such  as  dextrose 
and  Icevulose. 

3.  Proteolytic  enzymes : 

{a)  Pepsin 

Gastric 

Acid 

Converts  proteids  into  al- 

juice 

bumoses  and  peptones. 

[b]    Trypsin 

Pancreatic 
juice 

Alkaline 

Do. 

4.  Steatolytic  enzymes : 

(a)  Steapsin 

Pancreatic 

Alkaline 

Converts    fats    into    fatty 

juice 

acids  and  glycerine. 

5.  C oagulating enzymes:- 

{a)   Ren n in 

Gastric 

Alkaline, 

Coagulates  milk. 

juice 

neutral, 
or  faint- 
ly acid 

'   Called  "  Succus  cntcricits.'''' 

^  There  is  a  ferment  as  yet  not  named  found  in  pancreatic  juice  which  also 
coagulates  milk;  similar  ferments  are  also  found  in  the  juices  of  many  plants. 

L 


146  Elementary  Physiology. 

The  action  of  ptyalin  on  starch  takes  place  in  successive 
stages.  At  first,  the  starch  is  converted  into  a  more  soluble 
form,  known  as  soluble  starch;  next,  a  body  giving  a  red 
coloration  with  iodine,  and  called  erythro-dextrin^  is  formed 
simultaneously  with  a  certain  amount  of  a  sugar,  called  maltose ; 
finally,  there  are  formed  gum-like  bodies,  called  dextri7is^  which, 
since  they  give  no  colour  with  iodine,  are  called  achroo-dextrins, 
and  a  larger  percentage  of  maltose.  Complete  conversion  into 
maltose  never  takes  place,  even  after  prolonged  action  of 
ptyalin.  This  action  on  starch  is  stopped  about  half  an  hour 
after  the  food  has  entered  the  stomach — as  soon,  that  is,  as  the 
acid  reaction  of  the  first  portions  of  gastric  juice  secreted  has 
destroyed  the  ptyalin.  The  saliva  contains,  besides  mucin  and 
ptyalin,  only  a  small  amount  of  inorganic  salts.  The  most 
remarkable  of  these  inorganic  salts  is  a  trace  of  potassmm 
sulphocyaiiide^  which  can  be  detected  by  allowing  a  drop  of 
saliva  to  mix  on  filter-paper  with  a  drop  of  very  dilute  ferric 
chloride,  when  a  blood-red  colour  is  usually  produced. 

The  digestive  secretion  of  the  stomach  is  termed  the  gastric 
juice.  This  secretion  is  furnished  by  an  immense  number  of 
minute  glands  lying  in  the  thick  mucous  coat  of  the  stomach, 
which  pour  their  secretion  out  directly  upon  the  mucous 
membrane.  There  are  two  types  of  gland  found  in  the 
stomach.  In  one  of  these  types  there  are  two  kinds  of  cell 
(see  Figs.  74,  75).  One  kind  of  cell,  which  is  by  far  the  more 
numerous,  lines  the  lumen  of  the  gland,  and  lies  centrally  to 
the  other  kind  ;  these  cells  are  called  chiefs  or  central  cells.  The 
other  kind  of  cell  occurs  at  intervals,  and  is  removed  from  the 
central  lumen  by  the  thickness  of  the  row  of  chief  cells ;  these 
cells  are  called  parietal,  or  oxyntic  cells.  This  type  of  gland  occurs 
at  the  cardiac  end,  and  also  over  the  greater  part  of  the  stomach ; 
it  is  known  as  the  cardiac  gland.  The  other  type  of  gastric 
gland  {pyloric  glajid)  contains  only  one  kind  of  cell,  resembling 
the  chief  cell  of  the  cardiac  gland.  It  is  also  distinguished  by 
having  a  much  longer  duct  and  shorter  alveoli.  These  glands 
are  confined  to  the  pyloric  end  of  the  stomach.  In  connection 
with  this  difference  in  structure  there  is  a  corresponding  dif- 
ference in  the  character  of  the  secretion  of  the  two  kinds  of 


Fig.  T3.. — A  pj-loric  gland,  from  a  section  of  the  dog's  stomach.     (Ebstein.) 
w,  mouth  ;  «,  neck;   tr,  a  deep  portion  of  a  tubule  cut  transversely. 

Fig.  74.— Section  of  the  gastric  mucous  membrane  taken  across  the  direction  of  the 

glands  (cardiac  part). 
b,  basement  membrane  ;  c,  central  cells  ;  o,  parietal  cells ;  r,  retiform  tissue  (with 

sections  of  blood-capillaries)  between  the  glands. 
Fig.  75.— a  cardiac  gland  from  the  dog's  stomach.     (Highly  magnified.)     (Klein.) 
d,  duct  or  mouth  of  the  gland  ;  b,  base  or  fundus  of  one  of  its  tubules.     On  the  rio^ht  the 
base  of  a  tubule  more  highly  magnified  ;  c,  central  cell ;  /,  parietal  cell.   ° 


148  Elementary  Physiology. 

gland.  The  two  most  important  constituents  of  the  gastric 
juice  are  pepsin  and  hydrochloric  acid,  and  it  has  been  shown 
that  both  these  constituents  are  secreted  by  the  cardiac  glands, 
and  only  pepsin  by  the  pyloric  glands.  Since  the  only  kind  of 
cell  of  the  pyloric  gland  is  very  similar  to  the  chief  cell  of  the 
cardiac  gland,  it  has  been  supposed  that  the  chief  cells  secrete 
pepsin,  and  the  parietal  or  oxyntic  cells,  the  hydrochloric  acid. 

The  gastric  juice  has  little  or  no  action  on  carbohydrates 
or  fats,  but  acts  energetically  on  proteids. 

The  chief  enzyme  is  pepsin,  which  dissolves  proteid,  and 
converts  it  into  substances  which  are  afterwards  readily 
absorbed  by  the  cells  lining  the  intestine.  There  are  several 
stages  in  the  peptic  digestion  of  proteids.  First  the  proteid  is 
dissolved  and  rendered  non-coagulable  by  heat,  by  conversion 
into  acid  albumin  ;  at  this  stage  it  is  precipitated,  if  the  solution 
be  neutraHzed  by  dilute  alkali.  Next  it  passes  into  more  soluble 
substances,  called  albumoses,  which  are  not  precipitated  on 
neutralizing,  and  differs  from  the  original  proteids  in  several 
chemical  tests.  Finally,  a  portion  of  the  albumose  is  converted 
into  peptone,  which  is  still  more  soluble  and  more  easily 
absorbed  by  the  intestinal  cells,  and  here  the  process  stops. 
The  whole  drift  of  the  chemical  changes  is  thus  the  prepara- 
tion of  a  more  soluble  material  which  can  be  more  readily 
absorbed. 

The  gastric  juice  contains  a  second  ferment,  called  rejinin, 
which  has  the  property  of  curdling  milk.  The  process  bears  a 
close  resemblance  to  the  clotting  of  blood  (see  p.  127);  in 
both  cases  an  unorganized  ferment  produces  the  clotting,  and 
in  both  cases  the  presence  of  a  calcium  salt  is  necessary.  The 
curdling  of  milk  is  probably  not  the  only  purpose  of  rennin 
in  the  stomach,  for  it  is  found  in  the  gastric  juice  of  fishes. 

By  the  movements  of  the  stomach  and  the  digestive  action 
of  the  gastric  juice  combined,  the  food  is  reduced  to  a  soup- 
like mass,  of  varying  consistency  according  to  the  nature  of  the 
food,  which  is  called  chyme.  The  chyme  has  a  strongly  acid 
reaction,  due  to  the  hydrochloric  acid  of  the  gastric  juice.  At 
intervals,  portions  of  it  pass  through  the  pylorus  into  the 
duodenum,  where  they  soon  become  mixed  with  those  alkaline 


Diet,  Digestion,  Absorption,  and  Metabolism.     149 

secretions — the  bile  and  pancreatic  y?«V^— which  are  poured 
into  the  intestine  by  two  ducts/  opening  close  to  each  other 
about  3  or  4  inches  below  the  pylorus.  By  admixture  with 
these  secretions,  and  with  the  alkaline  mucus  furnished  by  the 
goblet  cells,  the  chyme  soon  acquires  an  alkaline  reaction,  and 
undergoes  further  attack  by  the  unorganized  ferments  or 
enzymes  of  these  secretions,  which  are  all  most  active  in  an. 
alkaline  medium. 

'Y\\Q  pancreatic  juice  is  the  most  important  of  the  digestive 
secretions,  containing  as  it  does  three  distinct  enzymes,  each 
acting  on  a  different  one  of  the  three  great  classes  of  food-stuffs. 

Amylopsin  completes  that  conversion  of  starches  into 
maltose  and  dextrins  which  was  temporarily  arrested  during 
gastric  digestion.  Its  action  closely  resembles  that  of  ptyalin 
(see  p.  146),  but  is  more  rapid. 

Trypsin  finishes  the  digestion  of  proteids  which  was 
commenced  by  pepsin  in  the  stomach.  It  differs  from  pepsin 
in  that  it  acts  in  an  alkaline  medium,  and  in  that  its  action  is 
much  quicker  and  more  profound.  The  albumose  stage  is 
rapidly  rushed  through,  so  that  it  is  impossible  to  isolate 
albumoses  from  an  artificial  tryptic  digestion,^  in  the  same  way 
as  from  a  peptic  digestion.  Further,  if  the  action  of  trypsin  be 
allowed  to  proceed  the  peptone  formed  is  broken  up  into  a 
number  of  simpler  organic  bodies. 

Steapsin  acts  upon  the  fats  of  the  food  which  have  been 
previously  freed  of  their  enveloping  connective  tissue,  through 
the  action  of  the  pepsin  of  the  gastric  juice  upon  it.  It  splits 
a  portion  of  the  fat  up  into  fatty  acid  and  glycerine,  and  the 
fatty  acids  so  formed  combine  with  the  alkali  present  in  the 
intestine  to  form  soaps.  In  the  soapy  solution,  the  remainder 
of  the  fat  usually  becomes  suspended  in  fine  drops  forming  an 

'  In  man  the  two  ducts  usually  join  before  opening  into  the  intestine, 
and  then  open  by  a  common  orifice,  while  in  many  animals  there  are  two 
or  even  more  pancreatic  ducts. 

^  It  is  by  means  of  artificial  digestion  in  glass  vessels  that  the  properties 
of  the  several  digestive  fluids  described  in  the  text  have  been  discovered. 
Either  a  quantity  of  the  digestive  secretion  itself  or  an  extract  of  the  gland 
yielding  it,  is  allowed  to  act  on  a  portion  of  the  food-stuff  for  a  given  time 
under  favourable  conditions  of  temperature  and  reaction,  then  the  fluid  is 
analyzed  and  the  changes  in  it  observed. 


ISO 


Elementary  Physiology. 


emiilsion  just  as  the  fat  of  milk  does.  In  this  finely  divided 
form  the  fat  readily  undergoes  further  attack  by  the  steapsin, 
and  is  probably  all  converted  in  the  end  into  free  fatty  acids 
and  glycerine.  In  some  animals  all  the  free  fatty  acid  com- 
bines with  alkalies  to  form  soaps,  while  in  others  the  alkali 
present  in  the  intestine  is  insufficient  for  the  purpose,  and  the 
fatty   acids   which   are   insoluble   in  water   are    dissolved   by 

the  agency  of  the  bile.     In 


addition  to  the  bile  and  pan- 
creatic juice  there  is  an  alka- 
line fluid  secreted  over  all 
parts  of  the  mucous  mem- 
brane of  the  intestine  by 
small  glands  imbedded  in  it, 
which  is  called  the  succus 
enter icus.  The  succus  enter iciLs 
is  probably  in  great  measure 
secreted  by  an  immense  num- 
ber of  minute  glands  called 
the  crypts  or  glands  of  Lieher- 


FiG.  76. — Cross-section  of  a  small  fragment 
of  the  mucovis  membrane  of  the  intestine, 


including  one  entire  crypt  of  Lieberkiihn     z,/;/„.       Thp«;p  nrf^  Qinrnlp  hihn 
and  parts  of  three  others.      (Magnified    ^"/^^^'       -»-  ilGSe  are  SimpiC  lUDU- 

400  diameters.)   (Frey.)  lar  invaginations  of  the  surfacc 

a,  cavity  of  the  tubular  glands  or  crypts  ;  3,         _        , 

one  of  the  lining  epithelial  cells  ;  c,  the     OI       the      mUCOUS      membrane 
interglandular  tissue ;  rf,  lymph-cells.  t        j        -,1  ,•  n      / 

Imed  with  secretmg  cells  (see 
Fig.  76);  similar  crypts  are  found  in  the  large  intestine,  but 
in  these  mucin  (goblet)  cells  are  more  common  (see  Fig.  77). 
Succus  entericus  is  strongly  alkaline  in  reaction,  and  contains 
an  inverting  ferment  {invertin)  which  acts  on  the  double  sugars 
(saccharoses)  and  changes  them  into  simple  sugars  (glucoses). 
In  this  way  it  attacks  the  maltose  formed  by  the  action  of 
the  saliva  and  pancreatic  juice  on  the  starch  of  the  food,  and 
changes  each  molecule  of  it  into  two  molecules  of  dextrose. 
In  a  similar  fashion  it  converts  cane  sugar  into  a  mixture  of 
equal  parts  of  dextrose  and  Isevulose.^ 

The  chemical  changes  in  the  food  brought  about  by  the 

^  The   action  is  a  hydrolytic  one — that  is   to  say,   the   elements  of  a 
molecule  of  water  are  taken  up,  as  might  be  represented  by  the  equation— 


Diet,  Digestion,  A  bsorption,  and  Metabolism.     1 5 1 

agency  of  these  various  digestive  secretions  are  intended  to 
adapt  it  for  more  easy  absorption  by  the  cells  lining  the 
intestine,  by  rendering  it  more  soluble  and  diffusible  so  that 
it  can  more  readily  enter  these  cells. 

We  may  next  turn  out*  attention  to  this  process  of  absorption, 


Fig.  77. — A  gland  of  the  large  intestine  of  the  dog.     (From  Heidenhain  and  Klose.) 
a,  in  longitudinal ;  b,  in  transverse  section. 


and  it  will  be  well  in  the  first  place  to  consider  the  arrange- 
ment of  the  absorbing  structures.  The  digested  food  is  in  the 
first  instance  absorbed  by  the  'cells  lining  the  alimentary  canal, 
and  is  afterwards  passed  on,  when  it  has  undergone  some 
modification  in  these  cells,  to  the  tissue  underlying  them.  By 
far  the  greater  portion  of  the  food  is  absorbed  in  the   small 


152 


Elementary  Physiology. 


intestine.  The  mouth  and  oesophagus  are  lined  by  stratified 
epithelium  ^  resembling  in  structure  that  covering  the  body  ex- 
ternally (the  epidermis), 
only  softer  in  its  texture ; 
such  an  epithelium  is  un- 
suitable for  absorption, 
and  practically  none  takes 
place.  In  the  stomach 
this  stratified  epithelium 
is  replaced  by  columnar 
epithelium,  which  in  a 
single  layer  occupies  the 
interspaces  between  the 
gastric  glands  and  pene- 
trates into  them  as  a 
lining  epithelium  for  their 
ducts.  This  epithelium 
seems  more  suitable  for 
absorption,  but  the  sur- 
face of  the  stomach  is 
smooth,  and  hence  has 
a  much  smaller  area  than 
that  of  the  intestine  which 
vastly    increased    by 


is 

finger  -  like  projections 
upon  it  called  villi  (see 
Fig.  78).  Further,  the 
gastric  columnar  cells  are 
found  not  to  absorb  most 
substances  in  solution  in 
water,  or  only  to  absorb 
them  very  feebly  j  so  that 
only  a  small  share  in  the 
process  of  absorption  is 


Fig.    78.  —  Small     intestine,    vertical     transverse 

section  with  the  blood-vessels  injected.    (Heitz- 

mann.) 
F,  avillus;  G,  g'andsofLieberkuhn;  Af,  musculans     j-^'h-pi^     "Ky     t]->g      stomacll 

mucosce  ;    A,    areolar    coat;    R,   nng-muscle     "-^-^^^^      ^ :i  _ 

(circular  layer  of  muscular  coat);  L,  longitudi-     'Y\\Q,  intCStinC  is  alsO  Imcd 

nal  layer  of  muscular  coat ;  P,  peritoneal  coat. 

^  Stratified  epithelium  means  a  lining  or  coating  tissue  in  which  the 
cells  are  many  layers  thick  (see  Fig.  92,  p.  201). 


Diet^  Digestion^  Absorption,  and  Metabolism.     153 

throughout  with  columnar  epitheUum  in  a  single  layer,  except  at 
the  rectum,  where  it  again  becomes  stratified.  In  the  small 
intestine  the  surface  is  not  smooth,  but  is  covered  all  over  with 
minute  projections  like  the  piles  on  a  piece  of  velvet.     In  this 


Fig.  79.— Cross  section  of  a  villus  of  the  cat's  intestine.     (Hiehly  maenified.) 

(E.A.  S.) 

e,  columnar  epithelium  ;  g,  goblet  cell  ;  its  mucus  is  seen  partly  exuded  ;  /,  lymph- 
corpuscles  between  the  epithelium  cells;  b,  basement  membrane;  c,  blood-capillaries  ; 
in,  section  of  plain  muscular  fibres  ;  c/,  central  lacteal. 

way  the  surface  through  which  absorption  can  take  place  is  enor- 
mously increased,  and  the  rate  at  which  this  process  can  go  on 
is  made  correspondingly  more  rapid.     Each  of  these  little  pro- 


FiG.  80. — Portion  of  small  intestine  distended  with  alcohol  and  laid  open  to  show  the 
valvulse  conniventes.     (Brinton.) 


jections  is  called  a  villus.  The  structure  of  the  wall  of  the  in- 
testine, the  arrangement  of  the  villi  upon  it,  and  the  structure 
of  a  villus  are  shown  in  the  accompanying  illustrations.  The 
intestinal  wall,  like  that  of  the  stomach,  has  four  coats  which, 


154  Elementary  Physiology. 

enumerated  from  the  outside  to  the  inside  of  the  tube,  are 
called  the  serous  or  peritoneal,  the  muscular,  the  areolar  or 
submucous,  and  the  mucous  coats. 

The  serous  coat  surrounds  the  intestine  in  the  jejunum 
and  ileum,  except  at  a  narrow  interval  along  the  attached  or 
mesenteric  border,  where  it  passes  off  and  becomes  continuous 
with  the  two  layers  of  the  mesentery.  The  duodenum,  as  well 
as  portions  of  the  large  intestine  (ascending  and  descending 
colon),  are  only  partially  covered  by  peritoneum. 

The  muscular  coat  is  arranged  in  two  portions,  each  con- 
sisting of  many  layers  of  muscle  fibres ;  the  fibres  of  the  inner 
portion  are  arranged  circularly  round  the  tube  {circular  nmsctdar 
coat),  while  those  of  the  outer  portion  run  longitudinally  along 
the  intestine  {longitudinal  muscular  coat).  The  inner  or  circular 
portion  is  much  thicker  than  the  outer  or  longitudinal  portion. 
Between  the  two  muscular  coats  there  is  a  plexus  ^  of  nerve 
fibres,  known  as  Auerbach's  plexus.  A  similar  plexus,  but 
with  a  closer  meshwork  of  fibres  and  fewer  nerve  cells,  lies  in 
the  submucous  coat  (Meissner's  plexus). 

The  submucous  coat  consists  of  a  layer  of  connective  tissue 
underlying  the  mucous  coat,  and  closely  connected  with  it. 
The  blood-vessels  ramify  in  this  connective  tissue  before  pass- 
ing into  the  mucous  membrane. 

The  mucou^s  coat  is  bounded  on  the  surface  nearer  the  sub- 
mucous coat  by  a  layer  of  plain  muscular  tissue  called  the 
muscula7-is  mucosce,  and  its  intestinal  surface  is  lined  with 
columnar  epithelium,  which  covers  the  villi  and  the  interspaces 
between  them.  The  space  between  these  two  surfaces  is 
occupied  by  fine  connective  tissue  {retiform  tissue),  which 
supports  the  blood-vessels,  nerves,  lacteals,^  and  crypts  of 
Lieberkiihn,  as  well  as  the  villi,  into  the  interior  of  which  it 
penetrates.  In  the  meshes  of  this  tissue  great  number  of  lymph 
corpuscles  (that  is,  amoeboid  leucocytic  cells)  are  found. 

*  A  nerve  plexus  is  formed  by  a  number  of  nerve  branches  uniting  with 
one  another,  interchanging  fibres,  and  then  separating  again.  In  the 
plexuses  described  above,  which  can  only  be  made  out  with  the  microscope, 
there  is  formed  a  network  of  fibres,  in  the  crossings  of  which  ganglia  of 
nerve  cells  are  situated. 

^   Vide  infra,  p.  155. 


Diet,  Digestion,  Absorption,  and  Metabolism.     155 

A  villus  is  a  projection  of  the  mucous  membrane  into  the 
cavity  of  the  intestine.  It  is  visible  to  the  naked  eye,  and  the 
neighbouring  villi  are  so  closely  set  together  that  the  inner 
surface  has  a  velvety  appearance.  The  villi  are  an  arrange- 
ment intended  both  to  greatly  increase  the  absorbing  area  of 
the  intestine,  and  also  to  bring  the  absorbing  channels  nearer 
to  the  absorbing  cells  lining  the  intestine.  The  first  of  these 
two  purposes  is  further  aided  by  transverse  foldings  of  the 
mucous  membrane  as  a  whole ;  these  larger  folds,  which  are 
known  as  vahmlcs  co?mivejtfes,  are  shown  in  Fig.  80. 

Each  villus  is  covered  by  columnar  epithelium,  and  within 
this  lies  fine  connective  tissue,  of  which  the  meshes  are  filled 
with  lymph  cells  (lymphoid  tissue).  This  tissue  serves  to 
support  a  network  of  capillary  vessels  lying  just  beneath  the 
columnar  cells ;  one  or  more  lymphatic  vessels,  here  called 
lacteals,-"-  which  occupy  a  central  position  in  the  villus  (see 
Fig.  79) ;  and  a  few  strands  of  plain  muscular  fibres  which  run 
longitudmally  surrounding  the  lacteals,  and  when  they  contract 
shorten  the  villus  and  expel  the  contents  of  the  lacteal.^  The 
blood-supply  of  each  villus  is  usually  obtained  from  one  arteriole, 
which  passes  from  the  submucous  coat  through  the  umsailaris 
mucoscE.  to  the  base  of  the  villus.  It  runs  up  the  centre  of  the 
villus  parallel  to  the  lacteal  for  about  halfway,  and  then  breaks 
up  into  a  number  of  capillaries,  which  form  a  plexus  or  net- 
work all  over  the  villus,  lying  just  beneath  the  columnar  cells. 
The  blood  is  collected  from  this  network  by  one  or  two  venules, 
which  originate  near  the  tip  of  the  villus,  and  passing  down  to 
its  base  join  a  venous  plexus  situated  in  the  mucous  coat. 
From  this  plexus  the  blood  is  conveyed  by  veins  to  the  larger 
venous  branches  in  the  submucous  coat. 

The  capillary  blood-vessels,  which  receive  a  great  part  of 
the  absorbed  food  after  it  has  been  acted  upon  by  the  columnar 
cells,  thus  lie  in  the  most  advantageous  position  for  favouring 

^  Because  they  are  filled  during  fat  absorption  with  a  fine  emulsion  of  fat, 
which  has  a  milky  appearance  ;  this  stream  of  emulsified  fat  can  then  also 
be  seen  filling  the  lymphatics  in  the  mesentery,  A^hich  are  on  this  account 
termed  lacteals. 

-  These  fibres  are  derived  from  and  connected  with  the  muscularis 
viucoscc,  mentioned  above  as  underlying  the  mucous  coat. 


156  Elementary  Physiology. 

speedy  absorption.  These  capillaries  take  up  the  soluble 
materials  which  have  been  formed  by  the  digestion  of  the 
proteids  and  carbohydrates  of  the  food ;  but  the  absorbed  fat 
passes  them,  and  finally  enters  the  lymphatic  capillaries  or 
lacteals  at  the  centre  of  the  villus. 

The  proteids  and  carbohydrates  absorbed  by  the  blood 
capillaries  are  carried  by  a  large  vein,  called  the  portal  vein,  to 
the  liver,  and  are  there  modified  in  certain  ways  ^  before  being 
passed  on  by  the  hepatic  veins  (which  leave  the  liver)  into  the 
inferior  vena  cava,  and  so  into  the  general  circulation. 

The  fats  pass  into  the  central  lacteal  in  a  very  finely 
emulsified  condition, ^  forming  a  milky  fluid  called  chyle,  which 
is  gathered  up  by  larger  lacteals  lying  in  the  mesentery,  and  by 
these  carried  to  a  number  of  lymphatic  glands  (abdominal 
lymphatics)  lying  at  the  attachment  of  the  mesentery  to  the 
abdominal  wall. 

After  passing  through  these  glands,  the  chyle  is  carried  to 
the  thoracic  duct,  the  main  trunk  of  the  lymphatic  system  (see 
Figs.  44,  45,  pp.  66,  67),  which  passes  up  through  the  thorax, 
close  to  the  vertebral  column  on  the  left  side,  to  enter  the 
venous  system  in  the  neck  at  the  junction  of  two  large  veins  of 
the  neck  (jugular  vein)  and  shoulder  (subclavian  vein). 

All  three  classes  of  food-stuifs  after  absorption  thus  eventually 
reach  the  general  blood-stream  :  the  proteids  and  carbohydrates 
via  the  portal  vein,  liver,  and  hepatic  vein ;  the  fats  via  the 
lacteals,  abdominal  lymphatic  glands,  and  thoracic  duct. 

The  absorbed  food-stuffs  are  modified  in  various  ways  by 
the  columnar  cells  after  their  absorption,  and  also  in  the  liver 
on  their  way  to  join  the  general  circulation. 

The  absorption  by  the  columnar  cells  of  substances  in 
solution  from  the  intestine  is  not  by  any  means  purely  a 
physical  process  of  diffusion.  Many  substances  which  are 
very  soluble,  and  diffuse  readily  through  dead  or  through 
inorganic  membranes,  such  as  the  iron  salts,  are  refused 
admission  altogether  by  the  living  columnar  intestinal  cells; 
and  other  substances  which  diffuse  very  slowly,  such  as  albu- 

^    Vide  infra,  p.  165. 

2  The  so-called  "molecular  basis  of  chyle." 


Diet,  Digestion,  Absoi^ption,  and  Metabolism.     157 

moses,  peptones,  and  dextrins,  are  taken  up  greedily  by  these 
lining  cells.  The  process  of  absorption  is  hence  a  selective 
one,  and  depends  on  the  vital  activity  of  the  columnar  cell. 
Nor  does  the  columnar  cell  act  like  an  inert  membrane  by 
allowing  those  substances  which  it  does  absorb  from  the 
intestine  to  pass  through  it  unaltered  into  the  retiform  tissue 
underlying  it,  and  so  reach  the  portal  circulation  unchanged  in 
nature.  For  each  cell  is  a  minute  laboratory  in  w^hich  raw 
products  are  taken  in  at  the  end  next  the  intestinal  cavity,  in 
the  shape  of  digestion  products,  and  finished  materials,  very 
different  in  character,  are  turned  out  at  the  fixed  end  to  pene- 
trate into  the  retiform  tissue,  and  reach  the  intestinal  capillaries 
and  the  lacteal.  In  this  way  all  the  albumose  and  peptone 
with  which  the  columnar  cell  is  fed  from  the  intestine  is  con- 
verted into  coagulable  proteid  again,  such  as  is  found  in  blood 
plasma ;  for  during  digestion  no  albumose  or  peptone  is  found 
in  the  blood  of  the  portal  vein,  nor  any  proteid  dissimilar  to 
the  ordinary  proteids  of  blood.  The  carbohydrate  of  the 
intestine,  in  whatever  form  it  may  be  absorbed  by  the  columnar 
cell,  is  also  acted  upon  by  this  cell,  and  changed  into  dextrose 
or  grape  sugar  j  for  this  is  the  only  form  of  carbohydrate  found 
during  carbohydrate  digestion  in  the  portal  blood.  Similarly, 
fats  are  synthesized  again  from  the  fatty  acids  and  glycerine, 
or  from  the  soaps  and  glycerine,  as  which  they  w^ere  mainly 
absorbed  by  the  lining  cells,  back  to  neutral  fat.  For  even 
when  fatty  acids  are  given  as  food,  it  is  as  neutral  fats  that 
they  are  afterwards  found  in  the  thoracic  duct. 

The  columnar  cell  of  the  intestine  hence  plays  an  important 
part  in  the  process  of  assimilation — that  is,  in  converting  the 
products  of  digestion  into  other  products,  identical  with  those 
contained  in  the  blood. 

The  portal  vein  carries  the  blood  which  has  been  collected 
from  the  intestinal  capillaries  to  the  liver,  and  in  this  important 
organ  the  blood  undergoes  various  changes  which  affect  not 
only  the  new  constituents  derived  from  the  food  and  absorbed 
during  the  passage  through  the  intestinal  capillaries,  but  also 
those  constituents  which  were  already  present  in  the  blood  as 
it  entered  the  mesenteric  arteries  to  supply  these  capillaries. 


158 


Elementaiy  Physiology. 


The  Liver. 
The  liver  is  by  far  the  largest  gland  in  the  body,  and  weighs 
on  an  average  50  to  60  ounces  in  the  adult,  being  about  -^  part 
of  the  weight  of  the  body ;  but  it  is  proportionately  heavier  in 
early  life,  forming  about  y^  part  of  the  body  weight  at  birth.^ 
It  is  divided  by  a  fissure  into  a  right  and  a  left  lobe ;  of  these 
the  right  is  much  the  larger,  being  about  four  times  as  great, 
and  further  divided  on  its  under  and  posterior  surface  into  three 
secondary  lobes  by  smaller  fissures  (see  Fig.  81). 


iiliiiiiiiliiil^v^ 


Fig.  81. — The  liver  of  a  young  subject,  sketched  from  below  and  behind. 

R.L.,  right  lobe  ;  L.L.,  left  lobe  ;  L.S.,  lobe  of  Spigelius ;  L.C.,  caudate  lobe  ;  L.Q., 
quadrate  lobe;/,  portal  fissure;  ii./.,  umbilical  fissure;  g:.hL,  gall-bladder;  v.c.i-, 
vena  cava  inferior  ;  i-g.,  impressions  on  the  under  surface  of  the  left  lobe  corresponding 
to  the  stomach  ;  C,  position  of  the  cardia  of  the  stomach  ;  X,  surface  of  the  liver 
uncovered  by  peritoneum. 

Posteriorly  there  is  a  transverse  fissure  at  right  angles  to  the 
longitudinal  fissure  at  which  the  vessels  supplying  the  liver  with 
blood  enter.  The  liver  differs  from  all  other  glands  in  the 
body  in  that  its  chief  blood-supply  is  veno2is^  being  carried  to  it 
by  the  portal  vein  from  the  capillaries  of  the  stomach,  intestines, 

^  The  student  Mall  find  it  advantageous  to  accompany  this  description 
with  a  practical  examination  and  dissection  of  the  liver  of  a  sheep  or  pig. 


Diet^  Digestion,  Absorption^  and  Metabolism.     159 

pancreas,  and  spleen.  Since  the  liver  cells  cannot  be  sustained 
by  venous  blood  alo7ie^  any  more  than  the  lungs  can  be  by  the 
venous  blood  carried  to  them  for  aeration  by  the  pulmonary 
arteries,  a  supply  of  arterial  blood  is  carried  to  the  liver  by  an 
artery  (which  is  small  compared  to  the  size  of  the  liver)  called 
the  Iiepatic  artery^  just  as  in  the  case  of  the  lungs  a  supply  of 
arterialized  blood  is  carried  by  the  bronchial  arteries.  In  this 
way  a  sufficient  supply  of  oxygen  is  brought  to  the  liver  cells  to 
carry  on  the  oxidative  changes  going  on  in  them.  This  peculiar 
blood-supply  of  the  liver,  and  its  position  as  it  lies  interposed 
between  the  blood  coming  from  the  alimentary  canal,  charged 
with  absorbed  products,  and  the  general  circulation,  show  that 
an  important  office  of  the  liver  is  to  produce  chemical  changes 
in  the  portal  blood  before  it  passes  on  to  the  general  circulation 
again.  Some  materials  are  taken  up  by  the  liver  cells  from  the 
blood ;  from  these  substances  others  are  formed,  and  these  new 
substances  are  restored  to  the  blood;  it  may  be  to  perform 
service  in  another  part  of  the  body  ;  it  may  be  in  suitable  form 
for  excretion  from  the  body  by  other  glands.  This  regulation 
of  chemical  changes  by  the  liver  in  building  up  some  sub- 
stances and  breaking  down  others  is  spoken  of  as  the  metabolic  ^ 
function  of  the  liver. 

The  blood  derived  from  both  the  portal  vein  and  hepatic 
artery  is  gathered  up  by  a  common  system  of  veins  after  it  has 
passed  through  the  liver  capillaries,  and  these  veins  unite  into 
larger  veins  called  the  hepatic  veins,  which  pour  their  stream 
into  the  inferior  vena  cava.  The  portal  vein  and  hepatic  artery 
enter  the  liver  together  at  the  transverse  fissure  (see  Fig.  81)  and 
are  accompanied  by  the  bile  duct.''-'     The  three  vessels  branch 

^  Metabolism  means  the  chemical  alterations  of  the  ingested  food  which 
go  on  in  the  body.  Further,  when  substances  are  built  up  or  synthesized  in 
the  body,  the  term  anabolisni  is  used  to  designate  the  process  ;  and  when 
substances  are  broken  up  into  simpler  ones,  and  energ}-  set  free  in  the  process. 
katabolism  is  tlie  term  used. 

-  The  bile  duct  has  different  names  in  different  parts  of  its  course :  that  part 
leading  from  the  liver  is  called  the  hepatic  duct ;  lower  down  this  branches, 
and  the  branch  going  to  the  gall  bladder  is  termed  the  cystic  duct ;  while  the 
portion  of  bile  duct  below  the  junction  leading  to  the  duodenum  is  called 
the  common  ML'  duct.  The  gall  bladder  is  a  distensible  bag  attached  to  the 
lower  surface  of  the  liver  into  which  the  bile  flows  at  intervals  when  it  is 
not  required  in  the  intestine,  and  from  which  it  is  discharged  when  an  in- 
creased supply  is  required  in  the  intestine  for  digestive  purposes. 


i6o 


Elementary  Physiology. 


together  as  they  subdivide  within  the  liver  and  are  surrounded 
by  a  common  sheath  of  loose  connective  tissue  known  as  the 


Fig.  82.— Section  of  a  portal  canal. 

a,  branch  of  hepatic  artery ;  v,  branch  of  portal  vein  ;  d,  bile-duct ;  /,  /,  lymphatics  in 

the  areolar  tissue  of  Glisson's  capsule  which  incloses  the  vessels. 

capsule  of  Glisson,  the  whole  being  termed  a  portal  canal  (see 
Fig.  82).  The  hepatic  vein  does  not  accompany  these  vessels, 
but  takes  a  separate  course  through  the  organ. 


Fig.  83. — Diagrammatic  representation  of  two  hepatic  lobules. 
The  left-hand  lobule  is  represented  with  the  "intralobular  vein  cut  across  ;  in  the  right- 
hand  one  the  section  takes  the  course  of  the  intralobular  vein.  /,  znUr\ohu\sir 
branches  of  the  portal  vein;  k,  i7ttra\oh\i\ar  branches  of  the  hepatic  veins;  s,  sub- 
lobular  vein ;  c,  capillaries  of  the  lobules.  The  arrows  indicate  the  direction  of  the 
course  of  the  blood.     The  liver-cells  are  only  represented  in  one  part  of  each  lobule. 


Diet,  Digestion,  Absorption,  and  Metabolism. 


i6i 


The  liver  substance  is  made  up  of  colonies  of  cells,  forming 
small  polyhedral  masses  called  lobules  (see  Fig.  84).  The  liver 
lobules  are  visible  to  the  naked  eye,  and  they  may  be  distmctly 
seen  on  the  fresh  liver  (especially  in  the  case  of  pig's  hver, 
where  the  lobules  are  more  separated)  as  hexagonal  spots,  each 


Fir    8a  —Section  of  a  portion  of  liver  passing  longitudinally  through  a  considerable 
hepatic  vein,  from  the  pig  (after  Kiernan).     (About  5  diameters.) 
H    heoatic  venous  trunk,  against  which  the  sides  of  the  lobules  are  applied  ;  h,  h,  h 
'    three  sublobular  hepatic  veins,  on  which  the  bases  of  the  lobules  rest    and  through 
coats  of  which  they  are  seen  as  polygonal  figures  ;  /,  mouth  of  the  nitralobular  veins, 
opening  into  the  sublobular  veins  ;  z',  intralobular  veins  shown  passing  up  the  centre 
Sf  some  divided  lobules ;  c,  c,  walls  of  the  hepatic  venous  canal,  with  the  polygonal 
bases  of  the  lobules. 

about  the  size  of  a  pin's  head.  Each  lobule  is  made  up  of  a 
large  number  of  liver  cells,  which,  like  the  lobules  themselves, 
and  from  a  similar  cause,  viz.  mutual  pressure,  are  polyhedral 
in  shape.  The  liver  capillaries  mn  among  these  cells  in  a 
manner  which  will  be  understood  by  referring  to  Fig.  83.  The 
capillaries  arise  from  the  ultimate  branches  of  the  portal  vein,^ 

1  The  hepatic  artery  supplies  the  capsule  of  the  liver,  the  portal  canal, 
and  the  walls  of  the  vessels  lying  therein,  and  also  has  interlobular  branches, 
which  are  much  smaller  than  those  of  the  portal  vein. 

M 


1 62  Elementary  Physiology. 

which  run  round  between  the  lobules,  and  are  hence  termed  inter- 
lobular veins.  The  capillaries  course  inward  to  the  centre  of 
each  lobule,  where  they  unite  to  form  an  intralobular  vein,  and 
the  intralobular  veins,  after  leaving  the  lobules,  open  into  the 
siLblobular  veins  (see  Fig.  84),  which  are  branches  of  the 
hepatic  vein. 

The  liver  cells  are  large,  and  distinct  in  their  outline  ;  they 
vary  in  appearance  according  to  the  condition  of  the  animal, 
being  clear  during  a  period  of  hunger,  but  full  of  granules  in  a 
well-fed  animal,  especially  a  few  hours  after  a  meal.  Some  of 
the  granules  consist  of  a  reserve  form  of  carbohydrate  termed 
glycogen,  which  the  liver  cells  form  from  any  excess  of  sugar 
which  may  be  present  in  the  blood  as  it  comes  to  the  liver. 
These  granules  stain  dark  brown  with  iodine.  Others  of  the 
granules  are  fat  globules  ;  for  the  liver  cells  also  act  as  a 
temporary  storehouse  for  fats. 

The  bile  ducts,  which  collect  the  bile  and  carry  it  away  from 
the  liver,  commence  between  the  hepatic  cells  as  fine  passages, 
or  canaliculi.  The  bile  canaliculi  open  into  the  minute  bile 
ducts  at  the  circumference  of  the  lobule,  and  these  unite  with 
one  another  to  form  larger  ducts 

The  Bile. 

Bile  is  a  thick  mucous  fluid  varying  in  colour  from  brown 
or  orange  yellow  to  olive  green.  The  viscidity  of  bile  is  due 
to  mucin,  which  is  added  to  it  in  the  gall-bladder.  The  mucin 
may  be  removed  as  a  stringy  precipitate  by  the  addition  of  a 
few  drops  of  acetic  acid,  and  the  filtrate  then  forms  a  mobile 
fluid.  Bile  contains,  in  addition  to  mucin,  the  following  con- 
stituents in  solution  in  water :  viz.  bile  salts,  bile  pigments, 
small  quantities  of  fats,  lecithin,  cholestearin,  and  inorganic 
salts. 

The  bile  salts  are  sodium  salts  of  two  complex  organic  acids 
called  glycocholic  and  taurocholic  acid.  These  are  compounds 
of  an  acid  called  cholalic  acid  with  an  amido-acid.  In  the  case 
of  glycocholic  acid  the  amido-acid  is  glycocoll  (amido-acetic 
acid),  and  in  the  case  of  taurocholic  acid  it  is  taurine  (amido- 
oxyethyl  sulphonic  acid).     An  amido-acid  is  one  in  which  an 


Diet,  Digestion,  Absorption,  and  Metabolism.     163 

atom  of  hydrogen  has  been  replaced  by  a  molecule  of  ammonia, 
and  hence  both  glycocoll  and  taurine  contain  nitrogen ;  taurine 
further  contains  sulphur.  The  presence  of  nitrogen  and 
sulphur  in  these  bodies  points  out  that  they  are  products  of  the 
decomposition  (katabolism)  of  proteids,  and  hence  that  the 
liver  in  which  they  are  formed  has  an  important  influence  on 
proteid  metabolism. 

The  bile  salts  have  an  important  function  in  rendering 
cholestearin  and  lecithin  soluble  in  the  bile,  and  so  providing  a 
means  for  the  removal  of  these  substances  from  the  body. 
Cholestearin  and  lecithin  are  decomposition  products  of  the 
nervous  tissues ;  they  are  insoluble  in  water,  but  by  the  agency 
of  the  bile  salts  are  rendered  soluble  in  the  bile,  and  so  can 
easily  be  conveyed  to  the  intestine  in  solution  and  removed 
from  the  body.  As  further  evidence  of  this  use  of  the  bile  salts 
there  is  the  fact  that  they  are  re-absorbed  in  great  part  from 
the  intestine  and  are  again  excreted  by  the  liver,  thus  completing 
what  is  known  as  the  circidatioii  of  the  bile.  A  similar  purpose 
is  served  by  the  bile  salts  in  the  absorption  of  fatty  acids  and 
soaps  from  the  intestine,  for  the  solubility  of  these  is  much 
increased  by  the  presence  of  bile  salts. 

The  bile  salts  have  an  intensely  bitter  taste,  and  may  further 
be  recognized  in  solution  by  the  intense  violet  they  give  when 
warmed  in  a  thin  film  with  a  drop  of  strong  sulphuric  acid  and 
a  crystal  of  cane  sugar  [Fettenkofer  s  test). 

The  bile  owes  its  colour  to  the  bile  pigments.  Two  pig- 
ments are  usually  present  in  bile,  in  varying  quantity,  called 
bilirubin  and  biliverdin.  Bilirubin  has  a  golden  yellow  colour, 
and  biliverdin  a  deep  olive  green,  so  that  the  varying  colour  of 
bile  is  due  to  the  different  proportions  in  which  different 
samples  contain  the  two  pigments.  The  two  pigments  are 
closely  related  in  chemical  composition,  biliverdin  being  an 
oxidized  derivative  of  bilirubin.  When  yellow-coloured  bile  is 
electrolyzed  it  turns  green  round  the  positive  pole  on  account 
of  oxidation  taking  place  there  ;  the  same  effect  is  obtained  if 
it  be  made  more  strongly  alkaline  and  exposed  to  the  oxygen 
of  the  air  in  thin  layers;  or  if  it  be  exposed  to  a  gentle 
oxidizing  agent   such   as  iodine   solution.     When   a    stronger 


164  Elementajy  Physiology. 

oxidizing  agent  is  employed,  such  as  strong  nitric  acid,  the  oxida- 
tion proceeds  further,  and  after  biHverdin  a  blue  coloured  body 
is  formed  {bilkyanin),  which  is  finally  replaced  by  a  dark  brown 
substance  called  choletelin. 

On  account  of  these  successive  oxidations,  when  a  thin  film 
of  bile  is  spread  out  on  a  porcelain  vessel  or  on  a  piece  of 
filter-paper,  and  a  drop  of  strong  fuming  nitric  acid  added  in 
the  centre,  a  change  of  colours  is  observed — first  green,  then 
blue,  and  finally  dark  brown.  These  colours  form  rings  round 
the  central  spot,  the  green  being  farthest  removed  from  the 
acid.  These  colour  changes  constitute  GineliiUs  test  for  the  bile 
pigments. 

Bilirubin  is  the  most  reduced  of  the  naturally  occurring  bile 
pigments,  but  when  it  is  acted  upon  by  a  reducing  agent  (such 
as  sodium  amalgam)  it  yields  an  artificial  compound  called 
hydrobilirubin.  The  importance  of  this  hydrobilirubin  lies  in 
the  fact  that  it  has  also  been  obtained  by  acting  with  reducing 
agents  on  hsematin,  a  decomposition  product  of  haemoglobin, 
and  hence  shows  that  the  bile  pigments  are  products  of  a  down- 
ward metabolism  of  haemoglobin  taking  place  in  the  liver. 
This  relationship  of  the  bile  pigments  to  haemoglobin  is  further 
shown  by  the  fact  that  in  old  blood-clots  formed  abnormally  in 
the  blood,  the  haemoglobin  of  the  red  blood  corpuscles  is  con- 
verted into  a  body  called  hcsjnatoidin,  which  has  been  shown  to 
be  identical  with  bilirubin.  The  bile  pigments  are  not  present 
in  large  amount  by  weight  in  the  bile,  and  but  for  this  connec- 
tion with  haemoglobin  would  not  have  any  great  physiological 
importance  ;  they  are  not  absorbed  again  like  the  bile  salts,  but 
pass  out  with  the  faeces.  In  the  passage  along  the  intestine 
they  become  completely  reduced,  and  are  present  in  the  faeces 
as  hydrobilirubin,  which  when  it  was  first  discovered  here  was 
termed  stercobilin. 

Bile  has  a  strongly  alkaline  reaction,  due  to  the  sodium 
carbonate  and  phosphate  which  it  contains ;  sodium  chloride 
is  the  only  other  inorganic  salt  which  it  contains  in  appreciable 
quantity. 


Diet^  Digestion,  Absorption,  and  Metabolism.     165 

Metabolism  in  the  Liver. 

We  have  next  to  consider  the  chemical  action  of  the  Uver 
on  the  blood  flowing  through  it.  The  function  of  the  liver  is 
here  twofold  in  character ;  in  the  first  place  its  cells  act  as  a 
temporary  storehouse  for  certain  classes  of  food-stuff,  and  in 
the  second,  the  final  stages  in  the  degradation  or  katabolism  of 
other  food-stuffs  take  place  here,  yielding  products  for  excretion 
from  the  blood-stream,  either  by  the  liver  itself  or  by  the 
kidneys. 

There  is  no  evidence  that  the  liver  acts  as  a  temporary 
storehouse  for  proteids ;  such  evidence  would  be  most  difficult 
to  obtain,  because  the  protoplasm  of  the  liver  cells  is  largely 
made  up  of  proteid,  and  hence  a  variation  in  the  amount  of 
proteid  would  be  hard  to  estimate.^  On  the  other  hand,  there 
is  clear  evidence  that  the  liver  cells  are  concerned  in  the 
katabolism  of  proteid.  This  is  shown,  not  only  by  the  identity 
of  the  bile  pigments,  as  stated  above,  with  the  degradation 
products  of  haemoglobin,  but  by  the  fact  that  urea.,  as  which 
practically  all  the  nitrogen  of  the  proteid  of  the  food  is  removed 
from  the  body,  is  formed  in  the  liver. 

Urea  is  excreted  by  the  kidneys  in  the  urine,  and  it  might 
be  supposed  at  first  sight  that  urea  was  formed  by  the  kidney 
cells,  but  this  has  been  shown  not  to  be  the  case.  When  the 
kidneys  of  an  animal  are  removed,  the  production  of  urea  in 
its  body  does  not  cease;  on  the  other  hand,  the  percentage 
of  urea  in  the  blood  increases,  and  the  animal  dies  from  poison- 
ing of  its  blood  with  urea.  This  shows  that  the  kidneys  do 
not  form  urea,  but  merely  remove  it  from  the  blood  passing 
through  them.  If  the  liver,  instead  of  the  kidneys,  be  removed, 
there  is  no  accumulation  of  urea  in  the  blood,  but  instead 
certain  ammonium  salts  make  their  appearance.  Further,  if 
the  blood  from  the  portal  vein  of  a  recently  fed  animal  be 

^  There  never  takes  place  in  the  body  in  general  so  much  storage  of 
proteid  as  there  does  of  carbohydrate  or  fat  ;  this  is  shown  by  the  fact  that 
when  the  proteid  in  the  food  is  increased,  the  nitrogen  excreted  as  i;rea  is 
correspondingly  increased.  Under  such  conditions,  the  excess  of  proteid  is 
not  used  to  form  tissue,  but  as  a  source  of  energy,  and  a  saver  of  carbo- 
hydrate and  fat. 


1 66  Elementary  Physiology. 

passed  through  the  excised  Hver  of  an  animal  which  has  not 
been  recently  fed  before  death,  it  is  found  that  the  percentage 
of  urea  in  the  blood  increases.  Again,  if  an  ammonium  salt 
be  added  to  whipped  blood,  which  is  then  perfused  through  a 
liver  just  excised  from  an  animal,  it  is  found  that  a  great  deal 
of  the  ammonia  disappears  and  is  replaced  by  urea.  It  is  clear 
from  these  experiments  that  the  liver  cells  produce  urea,  and 
that  they  probably  manufacture  it  from  ammonium  salts  cir- 
culating in  the  blood.  ^ 

It  was  at  one  time  believed  that  the  amount  of  urea  so  formed  represented 
the  tissue  waste,  and  hence  that  the  amount  of  urea  formed  and  excreted 
must  vary  directly  as  the  amount  of  work  done  by  the  tissues.  This  is  not, 
however,  the  case,  the  amount  of  urea  excreted  is  directly  proportional  to 
the  amount  of  proteid  food  consumed.  Of  course,  increased  work  by  the 
tissues  increases  the  amount  of  wear  and  tear  on  the  protoplasm  of  their 
cells,  and  hence  increases  the  amount  of  tissue  repair  required,  but  this 
amount  is  insignificant  compared  with  the  amount  of  proteid  or  other  food- 
stuff broken  down  to  supply  energy  for  muscular  contraction.  Carbohydrate 
and  fat  can  replace  proteid  as  sources  of  muscular  energy  at  the  expense  of 
chemical  energy.  Hence  if  the  amount  of  carbohydrate  food  given  be 
sufficient,  and  there  be  no  increase  of  proteid  food,  increased  muscular  work 
can  be  done  with  little  or  no  increase  in  the  formation  of  urea,  i.e.  without 
increased  proteid  consumption.  There  is,  however,  increased  production 
of  carbon-dioxide  and  water,  which  indicates  combustion  of  carbohydrate 
to  supply  the  necessary  energy  for  the  muscular  work  done.  It  follows  that 
the  relative  amounts  of  urea,  of  carbon-dioxide,  and  of  water  formed, 
depend  on  the  relative  amounts  of  carbohydrate,  of  fat,  and  of  proteid 
taken  as  food,  and  upon  these  only. 

While  it  is  improbable  that  the  liver  acts  to  any  appreciable 
extent  as  a  storehouse  for  proteids,  there  is  no  doubt  whatever 
that  it  has  an  important  function  in  so  acting  with  regard  to 
carbohydrates. 

When  an  animal  is  liberally  fed  on  a  carbohydrate  food,^ 
the  cells  of  its  liver  soon  become  charged  with  granules  of  a 
carbohydrate  belonging  to  the  amylose  (polysaccharide)  group, 

^  The  most  probable  salt  is  ammonium  lactate,  for  lactic  acid  is  formed 
during  muscular  contraction,  and  it  is  also  in  the  muscles  that  most  oxida- 
tion of  proteid  takes  place  when  proteid  food  is  liberally  given  ;  the  ammo- 
nium lactate  so  formed  passes  to  the  liver,  and,  in  some  unknown  way,  the 
liver  cells  prepare  urea  from  it,  which  is  restored  to  the  blood,  to  be  again 
removed  in  the  kidneys  and  passed  out  in  the  urine. 

-  Such  as  rice,  potatoes,  or  carrots. 


Diet,  Digestion,  Absorption,  and  Metabolism.     i6y 

and  known  a.s g/ycogen,  or  animal  starch.  The  glycogen  granules 
can  be  stained  a  deep  brown  when  sections  of  the  liver  are 
treated  with  tincture  of  iodine  ;  also  the  glycogen  itself  can  be 
prepared  in  quantity  from  such  a  liver,  and  its  chemical 
properties  tested.^  After  death  it  quickly  changes  into  grape 
sugar;  a  similar  change  also  takes  place  during  the  life  of  the 
animal,  if  the  carbohydrate  supply  be  stopped  or  greatly 
diminished.  Glycogen,  besides  being  found  in  the  liver,  is 
also  found  in  lesser  quantity  in  the  muscles,  but  disappears 
after  the  muscles  have  been  fatigued  by  over-work,  being  then 
used  up  to  furnish  a  supply  of  energy.  During  a  period  of 
rest,  there  is  a  new  formation  of  glycogen,  which  again  becomes 
expended  in  the  next  period  of  activity. 

Storage  of  glycogen  in  the  liver  cells  takes  place  when  the 
percentage  of  dextrose  in  the  portal  blood  coming  to  them 
exceeds  a  certain  limit ;  as,  for  example,  during  the  digestion 
of  a  meal  containing  carbohydrate.  On  the  other  hand,  when 
the  amount  of  dextrose  in  the  portal  vein  falls  below  a  certain 
limit  (about  2  parts  per  1000),  there  is  an  insufficient  amount 
of  circulating  carbohydrate  to  supply  the  necessary  energy  to 
the  tissues  (especially  to  the  muscles),  and  accordingly  a  certain 
amount  of  the  carbohydrate  previously  stored  in  the  liver  cells 
in  the  form  of  glycogen  is  reconverted  into  dextrose,  and 
discharged  into  the  b lood- stream  ^  to  keep  the  percentage  of 
circulating  carbohydrate  up  to  the  normal  mark.  These  state- 
ments are  supported  by  the  experimental  observations  that, 
during  digestion  of  carbohydrate  the  percentage  of  sugar  (dex- 
trose) in  the  portal  vein  (going  to  the  liver)  is  greater  than  that 
in  the  hepatic  vein  (leaving  the  liver) ;  while, 'during  a  period 
when  no  digestion  is  taking  place,  the  situation  is  reversed, 
and  there  is  more  sugar  in  the  hepatic  blood  than  in  the  portal 
blood.  Thus  the  liver  cells  act  as  governors  on  the  amount  of 
soluble  carbohydrate  in  the  form  of  dextrose  circulating  in  the 
blood.  This  is  an  important  function,  for,  on  the  one  hand, 
excess  of  dextrose  in  the  circulation  acts  injuriously  on  the 
tissues,  and  the  excess  is  treated  as  a  foreign  substance,  and 

*  See  Appendix. 

^  Not  directly,  of  course,  but  by  the  medium  of  the  intervening  lymph. 


1 68  Elementary  Physiology. 

excreted  by  the  kidneys,  giving  rise  to  a  great  waste  of  the 
carbohydrate  food ;  while,  on  the  other  hand,  an  insufficient 
amount  of  circulating  carbohydrate  leads  to  debility  of  the 
muscular  tissues,  and  to  morbid  changes  in  the  cells  of  the 
tissues  generally. 

This  storing  of  carbohydrate  is  spoken  of  as  the  glycogenic 
function  of  the  liver. 

This  function  may  be  disturbed  by  certain  artificial  means,  such  as  removal 
of  the  entire  pancreas  ;  puncture  of  a  portion  of  the  brain,  known  as  the  fourth 
ventricle  ;  administration  of  certain  drugs,  such  as  phloridzin,  or  curare. 
When  so  disturbed,  the  liver  cells  do  not  behave  any  longer  in  a  normal 
fashion ;  there  appears  an  excess  of  sugar  in  the  blood,  which  is  passed  out 
of  the  body  in  the  urine  [glycosuria^  diabetes)^  and  wasting  of  the  tissues  is 
the  result.  Under  such  circumstances,  complete  stoppage  of  carbohydrate 
in  the  food,  although  it  diminishes  the  amount  of  sugar  excreted,  does  not 
entirely  stop  it ;  for  the  liver  cells  continue  to  form  an  excessive  amount  of 
carbohydrate  from  the  proteid  of  the  food,  which  they  are  able  to  act  upon 
and  convert  in  part  into  carbohydrate. 

The  liver  cells  are  also  capable  of  acting  as  a  temporary 
storehouse  for  fats ;  for,  after  a  meal  rich  in  fat,  the  cells  of  the 
liver  are  found  to  contain  fat  granules.^  But  such  a  storage 
of  fat  in  the  liver  is,  under  normal  conditions,  very  transitory, 
and  prolonged  storage  takes  place  chiefly  in  the  connective 
tissue  of  certain  regions  of  the  body,  such  as,  in  the  sub- 
cutaneous connective  tissue  generally,  and  more  especially  in 
that  of  the  abdomen  ;  in  the  connective  tissue  lying  under  the 
peritoneum  in  which  the  kidneys  are  embedded ;  in  the  great 
omentum ;  and  upon  the  muscular  tissue  of  the  heart  under- 
neath the  pericardium.  In  these  parts  the  cells  of  the  con- 
nective tissue  become  loaded  with  fat,  which  first  appears  in 
the  cells  as  minute  granules.  These  granules  become  larger 
and  coalesce  until  the  cell  becomes  mainly  a  globule  of  fat 
surrounded  by  a  membrane  with  only  the  nucleus  and  a  trace 
of  the  cell  protoplasm  remaining.  Connective  tissue  so  altered 
to  store  up  fat  is  termed  adipose  tissue  ;  it  becomes  arranged  in 
lobules,  each  of  which  is  copiously  supplied  with  blood  by  a 
small  arteriole,  from  which  capillaries  are  formed  surrounding 

'  The  granules  are  shown  to  be  fat  by  their  staining  black  with  osmic 
acid,  and  by  their  solubility  in  ether  or  xylol. 


Diet,  Digestion,  Absorption^  and  Metabolism.     169 

the  fat  cells.  The  fat  cells  probably  synthesize  a  great  deal  of 
their  fat  from  carbohydrate  instead  of  from  fat,  and  it  is  also 
probable  that  when  there  is  a  scarcity  of  fatty  or  carbohydrate 
food,  and  the  storage  of  fat  comes  into  use,  that  part  of  it  is 
returned,  by  the  aid  of  the  cells,  to  the  blood  as  carbohydrate. 

In  the  animal  organism  it  is  certain  that  proteid  cannot  be 
formed  from  carbohydrate,  or  fat,  and  nitrogenous  inorganic 
salts.-^  Such  a  synthesis  can  only  be  carried  out  by  plant  cells, 
and  hence  upon  plant  proteid  directly  (herbivora)  or  indirectly 
(carnivora)  animal  life  is  dependant  for  its  indispensable  proteid 
supply.  With  this  one  reservation,  however,  it  may  be  stated 
that  the  three  classes  of  food-stuff  can  replace  one  another,  and 
can  be  converted  to  a  certain  extent  into  one  another  in  the 
course  of  the  complex  chemical  changes  which  go  on  in  the 
cells  of  an  animal's  body. 

Thus,  proteid  can  be  used  up  by  the  cells,  and  either  carbo- 
hydrate or  fat  produced  in  its  stead.  If  the  liver  of  an  animal 
be  freed  of  glycogen  by  administration  of  a  drug  (such  as 
phloridzin)  which  causes  the  glycogen  to  be  discharged  as 
sugar  in  the  urine,  and  then,  the  administration  of  the  drug 
being  stopped,  is  kept  on  a  purely  proteid  diet  for  some  time 
and  finally  killed,  it  is  found,  post-mortem,  that  the  liver  cells 
contain  a  fair  amount  of  glycogen.  Again,  in  cases  of  diabetes, 
as  stated  above,  the  excretion  of  sugar  in  the  urine  cannot  be 
completely  stopped  by  placing  the  patient  on  a  diet  from  which 
carbohydrates  are  carefully  excluded,  or  even  by  feeding  on 
proteid  alone. 

Similarly,  if  fat  be  excluded  from  the  food,  and  a  purely 
proteid  diet  be  given  to  an  animal,  in  a  short  time  the  liver 
cells  become  free  from  fat.  If,  at  this  stage,  phosphorus  be 
administered  to  the  animal,  and  it  be  killed  after  some  time,  it 
will  be  found  that  the  liver  cells  are  loaded  with  fat.  This 
production  of  fat  is  due  to  "  fatty  degeneration  "  of  the  proteid 
constituents  of  the  protoplasm  of  the  cells  under  the  influence 
of  the  drug.  Such  an  over-production  of  fat  from  proteid  is 
merely  an  exaggeration  of  a  normal  process,  and  it  is  probable 
that  when  an  excess    of  proteid   food   is    taken,  above    that 

'  This  is  shown  by  the  fact  that  animals  cannot  live  without  proteid  food. 


I/O  Elementary  Physiology, 

required  for  the  immediate  wants  of  the  animal,  that  a  portion 
of  the  excess  is  in  part  converted  by  chemical  changes  brought 
about  by  cell  protoplasm  into  carbohydrate  or  fat. 

There  is  also  abundant  evidence  that  carbohydrate  can  be 
converted  into  fat  and  stored  as  such  in  the  adipose  tissue  of 
the  body.  In  fact,  carbohydrate  food  is  a  more  efficient  fat 
producer  than  are  the  fats  themselves.  This  has  been  shown 
by  experiments  on  the  fattening  of  swine.  In  one  case  carbo- 
hydrate food  was  given,  and  in  the  other  a  corresponding 
weight  of  fat,  and  it  was  found  that  the  animals  fed  on  carbo- 
hydrate accumulated  much  more  fat  than  those  fed  on  fat. 
Further,  on  feeding  young  animals  on  a  food  rich  in  carbo- 
hydrates, and  containing  as  little  fat  as  possible,  it  is  found  that 
the  amount  of  fat  stored  up  in  the  body  is  much  greater  than 
the  total  amount  of  fat  found  by  analysis  as  having  been  given 
in  the  food. 

So  marked  is  this  formation  of  fat  from  carbohydrate  that  it 
was  at  one  time  thought  that  none  of  the  fat  taken  in  the  food 
could  be  directly  stored  in  the  body  without  alteration.  It  was 
supposed  that  all  the  fat  of  the  food  served  as  an  immediate 
source  of  energy,  for  muscular  work  and  for  heat  production 
(especially  the  latter) ;  that  all  the  fat  found  in  the  adipose 
tissue  was  synthetically  formed  from  other  material,  chiefly 
carbohydrate,  by  the  activity  of  the  cell  protoplasm ;  and  that 
none  could  be  directly  laid  down  without  change  from  the  fat 
of  the  food.  This  view  was  supported  by  the  fact  that  the  fat 
of  each  species  of  animal  has  a  fairly  definite  chemical  com- 
position and  melting-point,  due  to  the  admixture  of  the  several 
fats  composing  it  in  definite  proportions.  Thus,  pig's  lard  has 
a  lower  melting-point  than  the  fat  of  beef  suet,  and  this  again 
melts  more  easily  than  the  fat  of  mutton  suet.^  But  it  has  been 
shown  that,  tmder  certai7i  circumstances^  the  fat  of  the  food  may 
become  directly  incorporated  as  tissue  fat.      This  has  been 

^  These  fats  are  mixtures  of  three  fats,  called  olein,  palmitin,  and 
stearin  j  of  these  three,  olein  is  fluid  at  ordinary  atmospheric  temperatures, 
and  stearin  is  solid  at  even  several  degrees  above  body  temperature.  The 
melting-point  of  the  mixture  varies  with  the  proportion  of  each  present  ;  m 
lard  there  is  a  large  amount  of  olein,  in  mutton  fat  very  little,  and  hence 
lard  is  fluid  at  body  temperature,  while  mutton  fat  is  solid. 


Diet,  Digestion,  Absorption,  and  Metabolism.     171 

shown  in  two  ways.  One  consists  in  feeding  an  animal  for  a 
long  time  on  a  form  of  fat  containing  a  peculiar  constituent 
which  can  be  easily  recognized  again  by  chemical  methods,  and 
which  is  not  contained  in  the  ordinary  fats  of  the  animal's 
body.^  Afterwards,  when  the  animal  has  been  killed,  this 
peculiar  constituent  has  been  found  in  the  fat  of  its  body.  The 
other  method  consists  in  keeping  the  animal  on  a  low  diet  for 
some  time,  until  the  storage  of  fat  in  its  body  has  become 
greatly  reduced,  and  then  feeding  for  a  considerable  time  with 
fat  from  another  species  of  animal.  It  is  found  that  when  a 
dog  is  so  treated,  and  fed  on  mutton  suet,  that  the  fat  of  its 
body  has  a  much  higher  melting-point  than  is  normal  for  the 
fat  of  the  dog — approximating,  in  fact,  to  that  of  mutton  fat. 

In  order  to  obtain  a  successful  result  by  either  of  these 
methods,  it  is  necessary,  however,  that  the  stock  of  the  animal's 
own  fat  should  be  low,  that  it  should  be  allowed  to  eat  no 
other  kind  of  fat,  and  that  carbohydrates  should  also  be 
excluded  from  the  food.  These  are  somewhat  abnormal  con- 
ditions, and  hence,  although  the  experiments  demonstrate  that 
the  fat  of  the  food  may  be  directly  built  up  into  tissue  fat 
under  certain  conditions,  they  do  not  show  that  such  direct 
deposition  takes  place  to  any  marked  extent  under  normal  con- 
ditions. In  fact,  it  is  probable  that  under  usual  conditions  the 
greater  part  of  the  fat  of  the  body  is  synthesized  by  the  cells  of 
the  adipose  tissue  from  carbohydrate. 

To  sum  up,  then,  all  three  classes  of  food-stuffs  are  used 
in  the  body  as  sources  of  energy.  They  supply  the  cells  of 
the  various  tissues  with  nutriment,  and  undergo  chemical 
changes  in  these  cells  in  the  course  of  which  their  chemical 
energy  becomes  diminished,  being  converted  into  cell  activity 
and  in  the  end  into  heat,  which  serves  to  keep  up  the  temperature 
of  the  body.  Finally,  after  passing  through  intermediate  stages, 
they    are    resolved    into    bodies    of   much    simpler   chemical 

^  Fats  which  have  been  used  for  this  purpose  are  linseed  oil,  which  con- 
tains erucic  acid ;  and  spermaceti,  which  contain  cetyl  alcohol ;  these 
substances  are  not  present  in  the  ordinary  fats  of  the  food  and  of  the  body, 
and  their  presence  in  the  body-fat,  after  ingestion,  serves  as  a  signal  that 
the  fat  of  which  they  formed  a  constituent  has  not  been  broken  up  to  any 
great  extent  in  the  body. 


172  Elementary  Physiology. 

composition,^  and  possessed  of  but  little  chemical  energy,  which 
are  excreted  from  the  body  chiefly  by  the  lungs  and  kidneys. 
In  yielding  to  the  body  a  store  of  energy  the  different  food- 
stuffs can  replace  one  another,  but  a  certain  amount  of  proteid 
is  i7i  animal  life  indispensable,  because  this  form  of  food  alone 
can  repair  the  waste  of  protoplasm  going  on  in  the  tissues. 

^  The  chief  of  these  are  water  and  carbon  dioxide,  in  the  case  of  the  fats 
and  carbohydrates ;  and  in  addition  to  these  urea,  with  traces  of  other 
nitrogenous  bodies,  in  the  case  of  the  proteids. 


CHAPTER    VIII. 
RESPIRA  TION. 

In  the  processes  of  oxidation  which  are  continuously  going  on 

in  the  tissues,  oxygen  is  used  up  and  carbon  dioxide  formed.   The 

oxygen  is  carried  to  the  tissues,  and  the  carbon  dioxide  from 

the  tissues  by  the  blood.     Hence  the  blood,  in  passing  through 

the  systemic  capillaries,  becomes  poorer  in  oxygen  and  richer 

in  carbon  dioxide.     In  order  that  its  composition  may  remain 

unchanged  it  is  obvious  that  in  some  other  part  of  the  circuit 

it   must  take  up  oxygen  and  give   off  carbon  dioxide.     This 

change  takes  place  in  the  capillaries  of  the  lung,  in  its  passage 

through  which  the  blood  takes  up  oxygen  from  the  air  of  the 

lungs  and  gives  up  to  it  a  certain  amount  of  carbon  dioxide. 

In  passing  through  the  systemic  capillaries  the  arterial  blood 

becomes   venous;    in    the    passage    through    the   pulmonary 

capillaries  the  venous  blood  becomes  arterial.^ 

That  a  rapid  exchange  of  gases  may  take  place  in  the  lungs 

two    conditions   are  necessary  :    first,  that  there    should   be  a 

large  surface  of  blood  exposed  to  the  action  of  the  air  in  the 

lungs,  and  that  this  large  surface  of  blood  should  be  as  little 

as   possible    separated   by  tissue  from  the   air,  so  that   rapid 

diffusion  may  take  place ;  and,  secondly,  that  there  should  be 

some  means  of  quickly  changing  the  air  to  which  the  blood 

is   exposed,  so    that   it   may  not   become    charged  with   that 

gaseous  product  (carbon  dioxide)  which  it  is  essential  should 

be   removed   from   the   blood,  nor   poor   in   that   constituent 

(oxygen)   which  it  is  necessary  that   it   should  furnish  to  the 

blood. 

^  Hence  the  pulmonary  artery  contains  venous  blood,  and  the  pulmonary 
vem  arterial  blood. 


174 


Elementary  Physiology. 


Both    these    conditions    are   fulfilled    in   the    respiratory 
apparatus,  in  which  a  large  surface  of  capillaries  is  constantly 


Fig.  85. — The  trachea.     Front. 
h,  hyoid  bone  ;  ti ,   thyroid  cartilage  ;  c, 
cricoid  ;  e,  epiglottis  ;  tr,  trachea  ;  b 
and  3',  bronchi. 


Fig.  86.— The  trachea.     Back. 
a,  arytenoid  cartilages  ;  h,  hyoid  bone  ;  tt' , 
thyroid   cartilage  ;    c,    cricoid  ;    c,    epi- 
glottis ;  tr,  trachea  ;  b  and  b' ,  bronchi. 


exposed  to  air,  which  is  continually  renewed  by  the  respiratory 
movements  and  gaseous  diffusion. 

The  manner  in  which  the  air  is  alternately  pumped  into  and 


Respiration.  I75 

out  of  the  lungs,  on  account  of  the  lungs  being  compelled  to 
follow  passively  the  alterations  in  volume  of  the  air-tight  thorax, 
has  already  been  explained  (see  p.  32) ;  it  remains  to  describe 
the  structure  of  the  respiratory  system,  and  the  manner  in 
which  this  structure  facilitates  an  exchange  of  gases  between 
blood  and  air. 

The  respiratory  system  communicates  with  the  alimentary 
canal  at  the  front  and  lower  part  of  the  pharynx,  where  the 
windpipe  or  air-passage  begins  in  a  cartilaginous  box  called 
the   larynx,  which   is   the   organ    of  voice.      To   the   lower 


Fig.  87. — Diagrammatic  representation  of  the  ending  of  a  bronchial  tube  in  sacculated 

infundibula. 
B,  terminal  bronchus;  LB,  lobular  bronchiole  ;  A,  atrium  ;  I,  infundibulum  ;  C,  air- 
cells,  or  alveoli. 

end  of  the  larynx  the  trachea  is  attached  (see  Figs.  85  and  86)  j 
this  lies  in  front  of  the  oesophagus  in  the  neck,  and  enters  the 
thorax,  where  it  soon  bifurcates  into  two  branches  termed  the 
hronchi,  which  pass  one  to  each  lung.  The  bronchus  enters 
the  lung  at  the  root,  and  within  the  lung  branches  and  branches 
again,  in  a  tree-like  fashion,  giving  rise  to  the  bronchioles,  which 
become  smaller  in  section  at  each  successive  branching.  The 
ultimate  branches  are  known  as  the  bronchial  tubes,  each  of 
which  ends  in  an  expanded  sac-like  structure  known  as  the 
infundibulum.  The  walls  of  each  infundibulum  are  lined  by 
honeycombed  recesses  known  as  the  air  cells,  or  alveoli.  It 
is  in  these  minute  air-cells,  or  alveoli,  lining  the  walls  of  the 


176  Elementary  Physiology, 

infundibula  that  the  real  work  of  gaseous  exchange  between 
the  air  filhng  the  infundibula  {alveolar  aij-)  and  the  blood  in  the 
pulmonary  capillaries  takes  place.  It  will  be  readily  under- 
stood that  by  this  arrangement  a  very  large  extent  of  surface 
is  obtained.  By  far  the  greater  part  of  the  volume  of  a 
moderately  distended  lung  is  made  up  of  these  infundibula 
lined  with  alveoli,  so  that  the  total  surface  so  exposed  for  the 
aeration  of  the  blood  amounts  to  many  square  yards. ^ 

The  trachea,  bronchi,  and  bronchioles  are  held  open,  so 
as  to  always  allow  free  passage  to  the  air  in  and  out,  by 
incomplete  hoops  or  rings  of  cartilage  situated  circularly  in 
the  substance  of  their  walls.  The  cartilages  are  connected 
by  fibres  of  elastic  tissue  running  lengthwise  along  the  air- 
passages,  internal  to  the  cartilages,  and  between  them.  Ex- 
ternally there  is  a  coat  of  connective  tissue,  and  internally 
there  is  a  thick  mucous  membrane.  The  mucous  membrane  is 
lined  throughout  by  ciliated  epithelium  ^  from  the  upper  part 
of  the  trachea  to  the  termination  of  the  bronchioles  at  the 
commencement  of  the  infundibula  (see  Fig.  88).  Amongst 
the  ciliated  cells  there  are  situated  a  large  number  of  goblet 
cells  which  secrete  mucus  to  moisten  the  ciliated  mucous 
membrane.  The  cilia  move  in  such  a  way  as  to  move  any 
foreign  particles  which  may  have  been  drawn  in  with  the  air 
(as  well  as  the  mucus  moistening  them)  upwards  towards  the 
pharynx  and  so  away  from  the  lungs. 

In  this  manner,  the  lungs  are  greatly  protected  from  being  choked  with 
dust  and  floating  particles  of  all  kinds  drawn  in  with  the  air.  Still,  the 
protection  is  not  quite  complete,  and  some  of  the  finest  particles  float  in 
the  air  within  the  air-passages,  without  touching  the  walls  and  so  adhering 
to  them,  quite  down  to  the  alveoli,  where  there  is  no  ciliated  lining.  Such 
particles  become  absorbed,  and  in  the  case  of  old  persons  who  have  lived 
many  years  in  large  cities,  the  lungs  after  death  are  found  to  be  quite  black 
from  having  become  in  this  manner  impregnated  with  fine  coal  dust.^ 

^  The  alveolar  area  has  been  estimated  at  200  square  metres  (over  240 
square  yards). 

2  The  greater  part  of  the  larynx  (except  over  the  true  vocal  cords  and 
over  the  epiglottis)  is  also  lined  with  ciliated  epithelium. 

^  A  portion  of  this  foreign  matter  is  taken  up  by  the  numerous  lym- 
phatics of  the  lungs  and  carried  to  a  group  of  lymphatic  glands  situated 
at  the  root  of  each  lung  where  it  becomes  deposited,  so  that  after  some 


Respiration. 


177 


The  epithelial  cells  rest  on  a  membrane  {basement  jjiemhrane) 
which  separates  them  from  the  rest  of  the  mucous  coat  under- 


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lying  them  ;  this  consists  of  lymphoid  tissue  (that  is,  connective 
tissue  in  which  the   meshes  are  filled  by  lymph  corpuscles) 

years  these  dands  become  quite  oritty  ;  in  fact,  in  old  persons  who  have 
lived  in  cities,  more  like  pieces  of  coke  than  glands.  This  deposition  of 
carbon  is,  however,  quite  innocuous,  and  is  not  sufficient  to  interfere  materi- 
ally with  the  functions  of  the  lungs. 

N 


178  Elementary  Physiology. 

and  supports  numerous  small  blood  and  lymphatic  vessels  which 
supply  the  surrounding  tissue.  In  this  tissue,  as  well  as  deeper 
down  near  the  cartilaginous  rings,  in  what  is  termed  the  sub- 
mucous layer ^  lie  numerous  mucous  glands  (see  Fig.  88)  which 
open  on  the  ciliated  surface  and  assist  the  goblet  cells  in 
providing  the  moistening  mucous  fluid.  An  annular  ring  of 
muscle  fibres  of  the  plain  or  involuntary  variety  is  also  present, 
underlying  the  mucous  coat,  and  most  strongly  developed 
opposite  that  part  where  the  cartilage  is  incomplete.  In  the 
trachea  and  bronchi,  the  cartilages  are  all  incomplete  at  the 
same  part,  viz.  at  the  back,  and  here  there  is  a  strong  band 
of  involuntary  muscle  fibres  arranged  horizontally;  while  in 
the  bronchioles  the  incomplete  portions  of  the  rings  do  not 
correspond  in  situation.  The  advantage  of  the  incomplete- 
ness of  the  cartilaginous  rings  at  the  back  of  the  trachea  is 
obvious ;  here  the  trachea  lies  against  the  oesophagus  which 
is  usually  collapsed ;  but  when  a  bolus  of  food  passes  along, 
the  oesophagus  becomes  distended,  and  the  absence  of  the 
cartilages  in  front  where  the  trachea  lies  in  contact  with  it 
allows  the  necessary  distension  to  take  place  more  freely. 

In  the  smaller  branches  of  the  bronchioles  as  they  near 
the  infundibula  the  cartilage  disappears,  and  a  complete  layer 
of  plain  muscular  fibres  surrounds  the  tube,  the  whole  being 
enclosed  by  a  layer  of  loose  fibrous  tissue.  The  mucous 
membrane  lying  internal  to  the  muscular  layer  is  constituted 
much  as  in  the  larger  tubes,  but  there  is  less  connective  tissue. 
In  the  stratum  of  the  mucous  membrane  underlying  the 
epithelium  much  elastic  tissue  is  present,  of  which  the  fibres 
are  chiefly  arranged  parallel  to  the  length  of  the  tube.  As 
the  bronchial  tube  finally  expands  into  an  infundibulum,  the 
wall  thins  out,  and  the  several  layers  above  described  dis- 
appear; at  the  same  time  the  character  of  the  lining  epithelium 
alters,  and  the  ciliated  cells  are  chiefly  replaced  by  large 
irregularly  shaped  flattened  cells,  which  form  an  exceedingly 
thin  delicate  membrane  covering  the  alveolus  (see  Fig.  89). 
At  some  places  this  flattened  epithelial  layer  is  replaced  by 
cubical  epithelial  cells  (see  Fig.  89),  but  by  far  the  greater 
part  of  the  alveolar  surface  is  covered  by  the  flattened  scales, 


Respiration. 


179 


which  form  the  only  covering  (with  the  exception  of  the 
equally  thin  walls  of  the  capillaries  themselves)  separating  the 
blood  in  the  pulmonary  capillaries  from  the  air  in  the  alveoli. 

These  pulmonary  capillaries  are  arranged  in  a  close  mesh- 
work  over  the  concave  surface  of  each  alveolus,  just  beneath 
the  thin  pavement  epithelium.  The  pulmonary  artery  and 
its  larger  subdivisions  follow  the  branchings  of  the  bronchus 


Fig.  89.— Section  of  part  of  cat's  lung,  stained  with  nitrate  of  silver.     (Klein.) 
(Highly  magnified.) 
The  small  granular  and  the  large  flattened  cells  of  the  alveoli  are  shown.     In  the  middle 
is  a  section  of  a  lobular  bronchial  tube,  with  a  patch  of  the  granular  pavement- 
epithelium  cells  on  one  side. 


at  first,  but  the  finer  branches,  in  the  end,  leaving  the  smaller 
bronchioles,  branch  independently,  and  finally  small  arterioles 
are  given  off  from  these  which  run  round  the  margins  of  the 
alveoli  and  give  off  capillaries  all  the  way  round.  The 
capillaries  unite  to  form  minute  veins  which  collect  the  blood 
into  larger  venous  radicles  lying  in  the  connective  tissue 
between  the  infundibula,  and  these  unite  again  to  form  still 
larger  vessels,  which  after  pursuing  an  independent  course  for 


i8o 


Elementary  Physiology. 


some  time  finally  run  along  the  course  of  the  bronchioles,  and 
eventually  form  the  two  pulmonary  veins  which  leave  the  root 
of  each  lung  and  carry  the  oxygenated  blood  back  to  the  left 
auricle. 


Fig.  90.— Section  of  injected  lung,  including  several  contiguous  alveoli.    (F. 
Schultze.)     (Highly  magnified.) 


E. 


a,  a,  free  edges  of  alveoli ;  c,  c,  partitions  between  neighbouring  alveoli,  seen  in  section  ; 
b,  small  arterial  branch  giving  off  capillaries  to  the  alveoli.  The  looping  of  the 
vessels  to  either  side  of  the  partitions  is  well  exhibited.  Between  the  capillaries  is 
seen  the  homogeneous  alveolar  wall  with  nuclei  of  connective-tissue  corpuscles  and 
elastic  fibres. 


The  lungs  are  never  completely  emptied  of  air,  even  when 
the  greatest  effort  to  breathe  out^  is  made,  for  after  the  greatest 
possible  expiratory  effort  the  lungs  still  contain,  on  the  average, 
in  the  adult,  1000  cubic  centimetres  (about  60  cubic  inches) 
of  air ;  this  air  is  called  the  residual  air.  The  amount  of  air 
which  can  be  breathed  out  after  an  ordinary  expiration  down 


'  Breathing  out  is  termed  expit'ation,  and  breathing  in  is  termed  in- 
spiration. 


Respiration.  1 8 1 

to  the  greatest  possible  forced  expiration  is  termed  the  suppk- 
menfal  ov  reserve  ah;  and  it  measures  about  1500  cubic  centi- 
metres. These  two  fractions,  viz.  residual  and  supplemental  air, 
together  make  up  what  is  often  called  the  stationary  air,  because, 
in  quiet  breathing,  it  is  the  amount  retained  all  the  time  in  the 
lungs.  The  amount  of  air  normally  passing  in  and  out  of  the 
lungs  in  quiet  breathing  is  called  the  tidal  air,  and  measures  on 
the  average  about  300  cubic  centimetres,  although  it  varies  so 
in  different  individuals  that  the  average  amount  possesses  little 
importance.  Between  the  amount  of  air  in  the  lungs  at  the 
end  of  an  ordinary  inspiration  and  the  amount  at  the  end  of 
the  greatest  possible  inspiration  there  is  a  difference  of  about 
1700  cubic  centimetres,  and  this  frac-tion  is  termed  the  comple- 
mental  air. 

The  maximum  amount  of  air  which  can  be  taken  into  or 
breathed  out  from  the  lungs  by  a  single  effort,  obviously  includes 
the  fractions  of  reserve^  tidal,  and  complemental  air ;  this  amount 
is  known  as  the  vital  capacity,  and  measures  3000  to  4000  cubic 
centimetres. 

In  quiet  breathing,  the  air  enters  the  nostrils  and  passes 
along  the  nasal  passages  to  enter  the  pharynx  at  the  posterior 
openings  of  these  passages — the  posterior  nares.  It  passes 
through  the  pharynx,  enters  the  larynx  at  the  opening  of  this 
from  the  pharynx,  called  the  glottis,  and  passes  down  the 
trachea  into  the  bronchi.  The  amount  of  air  taken  in  at  each 
ordinary  inspiration  is  quite  insufficient  to  reach  the  alveoH, 
and  is  merely  enough  to  fill  the  nasal  passages,  phar^^nx, 
larynx,  trachea,  bronchi,  and  larger  bronchioles.  The  rest  of 
the  work,  whereby  the  alveolar  air  becomes  changed  in  com- 
position, is  effected  by  gaseous  diffusion,  and  so  perfect  and 
rapid  is  this  that  the  alveolar  air  differs  but  Little  in  composition 
from  expired  air. 

The  air  in  its  passage  through  the  air-passages  is  brought 
in  contact  with  the  warm  and  moist  surface  of  the  mucous 
membrane  lining  the  nasal  cavities,  and  is  here  both  warmed 
almost  to  the  tem_perature  of  the  body,  and  nearly  saturated 
with  water  vapour.  These  processes  are  completed  before  the 
air  leaves  the  body,  so  that  the  expired  air  has  the  temperatiire 


l82 


Elementary  Physiology. 


of  the  body,  and  is  completely  saturated  with  aqueous  vapour 
at  that  temperature.^ 

The  chemical  composition  of  the  air  is  also  altered  in  the 
process  of  respiration.     Atmospheric   air   contains   in   round 


-,5 


Fig.  91. — Medial  section  of  the  face  and  neck, 
sphenoid  bone;  2,  nasal  cavity  ;  3,  brain  cavity;  4,  ethmoid  bone  ;  5,  frontal  bone; 
6,  nasal  bone ;  7,  superior  maxillary  bone  ;  8,  palatal  bone  ;  9,  superior  turbinated 
bone  ;  10,  middle  turbinated  bone  ;  11,  inferior  turbinated  bone  ;  12,  soft  palate  ;  13, 
upper  part  of  pharynx  ;  14,  lower  part  of  pharynx  ;  15,  oesophagus  ;  16,  larynx  ;  17, 
glottis  ;  18,  epiglottis  ;  19,  opening  of  Eustachian  tube  ;  20,  inferior  maxillary  bone  ; 
21,  tongue  ;  22,  tonsil ;  a  to  f,  bodies  of  cervical  vertebrae  ;  s,  spinal  cord  ;  /,  pro- 
cesses of  cervical  vertebrae  ;  o,  portion  of  occipital  bone. 


numbers,  in  loo  parts  by  volume,  about  79  parts  of  nitrogen, 
21  of  oxygen,  and  but  3  parts  in  10,000  (in  good  air)  of  carbon 
dioxide,  while  expired  air  contains  about  4  per  cent,  of  carbon 
dioxide,  only  16  per  cent,  of  oxygen,  and  the  balance  of 
nitrogen.     The  presence  of  carbon  dioxide  in  expired  air  can 

^  See  p.  192. 


Respiration.  183 

easily  be  shown  by  breathing  out  through  lime  water,  when  a 
heavy  white  precipitate  of  calcium  carbonate  is  obtained.  By 
somewhat  more  delicate  analysis,  the  diminution  in  the  per- 
centage of  oxygen  may  be  detected;  this  diminution  in  the 
oxygen  is  always  in  excess  of  the  carbon  dioxide  formed,  and 
the  volume  of  the  expired  air  is  correspondingly  less  than  that 
of  the  inspired  air,  showing  that  the  oxygen  which  is  missing 
has  not  gone  to  form  a  gaseous  compound.  As  already 
explained,  this  oxygen  has  been  utilized  in  combining  with 
the  excess  of  hydrogen  in  the  fats  and  proteids  of  the  food 
during  the  combustion  of  these  bodies  in  the  tissues. 

The  changes  in  the  air  in  the  process  of  respiration,  then, 
are  these — 

{a)  The  air  is  warmed  (or  cooled)  to  the  temperature  of 
the  body. 

{b)  The  air  is  saturated  with  aqueous  vapour  at  that 
temperature. 

(c)  The  air  is  changed  in  chemical  cojnposition^  about  5  per 
cent. ,  roughly^  of  oxyge?i  disappearing  and  4  per  cent,  of  carbon 
dioxide  appearing. 

The  chemicnl  changes  which  give  rise  to  the  alteration  in 
chemical  composition  of  respired  air  do  not  take  place,  as  was 
originally  thought,  in  the  lungs,  but  /;/  the  tissues.  The  process 
is  really  one  of  slow  combustion,  or  chemical  oxidation,  going 
on  in  the  cells  of  the  tissues,  and  chiefly  in  the  muscular 
tissues,  and  the  lungs  are  a  ventilating  agency,  taking  in  stores 
of  oxygen  and  removing  the  carbon  dioxide  formed. 

That  the  oxidation  does  not  take  place  in  the  lungs  is  shown 
by  the  fact  that  the  arterialized  blood  leaving  the  lungs  contains 
a  much  higher  percentage  of  oxygen  than  the  venous  blood 
coming  to  the  lungs,  and  at  the  same  time  a  much  less  per- 
centage of  carbon  dioxide,^  thus  showing  that  the  blood  has 

*  Although  the  above  statement  is  true,  still  both  venous  and  arterial 
blood  contain  more  carbon  dioxide  than  oxygen.  Blood  contains  in  round 
numbers  60  per  cent,  of  gas,  of  M'hich,  in  arterial  blood,  about  40  volumes 
are  carbon  dioxide,  and  20  volumes  oxygen,  while  in  venous  blood  about 
46  volumes  are  carbon  dioxide,  and  8  to  12  volumes  are  oxygen.  Both 
arterial  and  venous  blood  contain  about  I  per  cent,  of  nitrogen,  which  is 
simply  dissolved  in  the  plasma. 


184  Elementary  Physiology. 

given  up  carbon  dioxide  in  the  lungs  and  taken  up  a  supply 
of  oxygen  for  use  somewhere  else  in  the  circuit.  That  the 
oxidation  does  not  take  place  in  the  blood  is  shown  by  the 
fact  that  the  composition  of  the  contained  gases  does  not 
change  until  the  capillaries  have  been  reached,  and  that  it 
does  take  place  in  the  tissues  is  shown  by  the  fact  that,  in 
passing  through  the  capillaries  of  the  tissues,  all  the  decrease 
in  oxygen  and  increase  in  carbon  dioxide  occurs. 

We  have  next  to  consider  in  what  way  oxygen  is  taken  up 
in  the  passage  through  the  lungs,  and  carbon  dioxide  given  off; 
why  the  reverse  change  takes  place  in  the  tissues,  and  how  the 
oxygen  and  carbon  dioxide  are  held  in  the  blood. 

The  oxygen  and  carbon  dioxide  are  held  dissolved  in  the 
blood  partially  by  physical  and  partially  by  chemical  means — 
that  is  to  say,  partially  in  solution  and  partially  in  chemical 
combinations. 

The  amount  of  oxygen  which  the  blood  plasma  is  capable 
of  holding  in  simple  solution  is  very  small,  and  is  only  a  small 
fraction  of  that  which  is  taken  up  by  the  blood ;  by  far  the 
greater  part  is  held  in  a  loose  state  of  chemical  combination 
with  the  hmmoglohin  of  the  red  corpuscles,  forming  an  unstable 
compound,  called  (?.r>'-haemoglobin.  When  the  oxygen  pressure 
in  the  plasma  is  high,  as  is  the  case  when  the  blood  is  in 
contact  with  air  containing  a  fair  percentage  of  oxygen  (as,  for 
example,  in  the  lungs),  then  the  haemoglobin  takes  up  oxygen 
from  the  plasma  until  it  becomes  saturated  with  it.  On  the 
other  hand,  when  the  oxygen  pressure  in  the  plasma  is  low, 
the  compound  of  haemoglobin  becomes  broken  up,^  and  the 
oxygen  is  given  out  to  the  plasma. 

This  taking  up  or  giving  out  of  oxygen  by  the  haemoglobin 
is  not  a  very  gradual  process,  the  amotmt  of  oxygen  taken  up 
does  not  mcrease  proportionately  to  the  pressure  of  oxygen,  hut 
at  a  certain  pressuj'e  of  oxygen  the  gas  is  taken  up  rapidly,  and 
when  the  press2L7t  is  only  slightly  higJur,  the  h(Enioglohin  becomes 
almost  completely  saturated,  a7id  takes  up  very  little  more  even 
if  the  oxygen  pressure  be  greatly  increased.      The  pressure  at 

^  The  oxy-haemoglobin  is  then  reduced  or  converted  into  reduced* 
haemoglobin. 


Respiration.  185 

which  the  haemoglobin  becomes  practically  saturated  is  low, 
being  considerably  less  than  half  that  of  the  oxygen  in  the 
alveolar  air.  The  oxygen  pressure  in  the  lymph  bathing  the 
tissues  is  very  low,  because  the  cells  of  these  tissues  are  con- 
tinually using  up  oxygen ;  and,  at  this  low  pressure,  the 
haemoglobin  rapidly  loses  oxygen  and  becomes  partially 
reduced. 

There  is  an  evident  advantage  in  the  oxy^gen  being  thus 
held  in  loose  chemical  combination,  for  when  a  gas  is  held  in 
physical  solution  in  a  fluid,  the  amount  (weight)  dissolved  is 
directly  proportional  to  the  pressure,  and  in  order  to  hold  the 
same  amount  of  oxygen  in  solution  in  the  plasma  as  can  be 
held  in  chemical  combination  in  the  haemoglobin,  the  pressure 
would  require  to  be  enormous,  so  that  it  would  be  impossible, 
under  existing  atmospheric  conditions,  to  keep  the  cells  supplied 
with  that  amount  of  oxygen  which  they  require. 

Both  in  the  lungs  and  in  the  tissues  the  plasma  in  which 
the  red  blood  corpuscles  float  plays  the  part  of  an  intermediary 
between  the  oxygen  and  haemoglobin.  The  venous  blood 
arriving  at  the  lungs  is  poor  in  oxygen,  and  this  poverty  is 
shared  by  the  plasma  and  the  corpuscles ;  part  of  the  haemo- 
globin is  in  a  reduced  condition,  the  amount  depending  upon 
the  pressure  of  oxygen  in  the  plasma.^  In  the  passage  through 
the  pulmonary  capillaries,  on  account  of  the  low  pressure  of 
oxygen  in  the  plasma,  a  small  amount  of  oxygen  is  first  dis- 
solved from  the  alveolar  air,  and  if  there  were  no  hjemoglobin, 
this  small  amount  would  rapidly  raise  the  pressure  of  oxygen  in 
the  plasma  to  that  of  the  oxygen  of  alveolar  air,  and  absorption 

^  The  pressure  of  a  gas  in  a  fluid  is  often  spoken  of  as  its  tension  ;  thus, 
for  example,  one  speaks  of  the  oxygen  tension  of  arterial  or  venous  blood  ; 
but  the  term  is  of  no  great  use,  for  this  tension  is  measured  by  the  pressure 
of  the  gas  over  the  fluid  when  fluid  and  gas  are  in  equilibrium,  and  gas 
is  neither  absorbed  nor  liberated,  and  hence  it  is  quite  correct  to  speak  of  the 
oxygen  pressure  in  this  sense.  When  the  pressure  on  the  surface  of  the 
fluid  of  the  dissolved  gas  is  diminished,  as,  for  example,  when  a  bottle  of 
soda  water  is  opened,  then  the  pressure  of  the  gas  within  the  fluid  is  greater 
than  its  pressure  outside  the  fluid,  and  gas  is  evolved  from  the  fluid  ;  and  on 
the  other  hand,  if  the  pressure  of  the  gas  on  the  surface  be  increased,  more 
gas  is  dissolved,  ^Yhen  a  mixture  of  gases  are  exposed  to  the  fluid,  the 
amount  of  each  taken  up  depends  on  its  own  pressure  in  the  mixture,  or,  as 
it  is  called,  iis  partial  pressi^re,  and  not  on  the  total  pressure  of  the  mixture. 


1 86  Elementary  Physiology, 

would  stop ;  but  as  the  pressure  of  oxygen  in  the  plasma 
increases,'  the  haemoglobin  begins  to  combine  with  some  of  the 
oxygen,  thus  causing  the  pressure  in  the  plasma  to  rise  more 
slowly.  So  with  a  much  smaller  change  in  pressure,  a  much 
larger  quantity  of  oxygen  is  taken  up.  Exactly  the  reverse 
change  takes  place  in  the  tissues,  for  here  oxygen  is  always 
being  used  up,  and  consequently  the  lymph  bathing  the  cells 
is  poor  in  oxygen,  and  the  oxygen  pressure  in  it  is  very  low. 
There  is  accordingly  a  diffusion  of  oxygen  from  the  plasma 
where  the  oxygen  pressure  is  high  into  the  lymph  where  the 
oxygen  pressure  is  low,  and  the  pressure  in  the  plasma  would 
soon  fall,  and  so  also  the  supply  of  oxygen  to  the  cells,  were  it 
not  that  the  lowered  pressure  cause  the  oxy-haemoglobin  to  split 
up  and  yield  a  fresh  supply  of  oxygen  to  the  plasma.  All  the 
oxygen  is  never  used  up  in  any  one  circuit  through  an  organ ; 
there  is  always  a  large  stock  of  reserve  oxygen  in  venous  blood, 
otherwise  respiration  could  not  be  stopped  for  an  instant 
without  causing  suffocation. 

The  passage  of  the  carbon  dioxide  takes  place  in  an  inverse 
direction.  The  carbon  dioxide  is  not  certainly  known  to  form 
any  definite  compound,  such  as  oxygen  does  with  the  haemo- 
globin. It  is  more  soluble  in  plasma  than  oxygen  is,  and  a 
certain  amount  is  held  in  solution ;  another  fraction  is  held  in 
chemical  combination,  in  part  as  sodium  bicarbonate,  in  part 
with  the  proteids  of  the  plasma,  and  also  probably  in  part  with 
the  corpuscles.  The  pressure  of  the  carbon  dioxide  in  the 
plasma  of  the  venous  blood  coming  to  the  lungs  is  higher  than 
its  pressure  in  the  alveolar  air,  and  hence  carbon  dioxide  is 
evolved,  lowering  the  amount  in  the  blood,  and  increasing  the 
amount  in  the  alveolar  air.  In  the  tissues,  the  lymph  is  highly 
charged  with  carbon  dioxide,  and  hence  there  is  a  diffusion 
stream  of  carbon  dioxide  into  the  plasma,  raising  the  pressure 
of  carbon  dioxide  in  it.  In  this  way  there  is  a  continuous 
cycle  of  change,  oxygen  is  taken  in  and  carbon  dioxide  thrown 
out,  in  the  lungs,  and  the  supply  of  oxygen  so  obtained  is 
borne  by  the  circulating  blood  to  the  tissues,  where  it  combines 
in  part  with  hydrogen  and  in  part  with  carbon.  That  part 
which  combines  with  carbon  gives  rise  to  carbon  dioxide,  which 


Respiration.  i  ^J 

diffuses  out  from  the  cells  to  the  lymph,  from  the  lymph  to  the 
plasma,  and  in  the  plasma  is  carried  to  the  lungs,  where  it  is 
removed. 

These  changes  may  also  be  observed  in  blood  outside  the 
body,  if  the  pressure  of  oxygen  and  carbon  dioxide  upon  its 
surface  be  varied. 

If  blood  which  has  been  whipped,  to  prevent  it  becoming 
solid  in  clotting,  be  placed  in  a  bulb  which  is  connected  with 
a  mercurial  air-pump,  and  the  air  within  the  bulb  removed, 
then  the  gases  contained  in  the  blood  are  given  off  into  the 
vacuum.  At  the  same  time,  as  the  blood  loses  its  oxygen  it 
changes  its  colour  from  bright  red  to  purple.  If  air  be  again 
admitted  to  the  bulb,  and  especially  if  the  blood  be  shaken  up 
with  it,  the  reverse  change  takes  place,  for  the  blood  absorbs 
oxygen  and  turns  back  to  red  in  colour. 

The  same  change  takes  place  when  blood  is  left  in  contact 
with  reducing  agents,  for  these  seize  the  oxygen  from  the 
oxy-haemoglobin,  and  reduced  hsemoglobin  is  formed.  Thus 
if  blood  be  shaken  up  with  ammonium  sulphide,  and  then 
allowed  to  stand  for  a  few  minutes,  the  ammonium  sulphide  is 
oxidized  at  the  expense  of  the  oxygen,  and  reduced  haemo- 
globin is  formed.  A  certain  amount  of  reducing  power  is 
possessed  by  the  blood  itself,  and  the  amount  of  reducing  sub- 
stances in  it  increases  on  standing.  Hence,  when  a  large  clot 
of  blood,  which  has  stood  for  some  time,  is  cut  into,  although 
it  is  red  on  the  surface  where  it  has  been  in  contact  with  the 
oxygen  of  the  air,  inside  it  is  dark  purple,  almost  black,  in 
colour.  Because  the  oxygen  of  the  air  has  not  been  able  to 
penetrate,  and  the  reducing  substances  formed  in  the  mass 
(or  the  oxidation  processes  going  on  there)  have  reduced  the 
oxy-haemoglobin. 

The  changes  from  the  arterial  to  the  venous  condition,  aiid 
back  again  from  venous  to  arterial,  may  also  be  shown  by 
bubbling  alternately  a  stream  of  carbon  dioxide,  and  one  of 
oxygen,  or  atmospheric  air,  through  some  whipped  blood. 
When  the  carbon  dioxide  is  passed  through,  it  carries  away  all 
the  oxygen,  and  the  blood  becomes  venous ;  when  the  air  is 
passed  through,  it  carries  off  the  carbon  dioxide,  and  leaves  a 


1 88  Elementary  Physiology, 

supply  of  oxygen  to  combine  with  the  haemoglobin,  so  that  the 
blood  becomes  arterial. 

Anything  which  prevents  the  combination  of  haemoglobin 
with  oxygen  in  the  lungs,  and  so  stops  the  supply  of  oxygen  to 
the  tissues,  endangers  the  life  of  the  animal.  And  if  the  supply 
be  stopped  completely  for  a  short  time  (two  to  five  minutes), 
or  be  insufficient  for  a  longer  period,  the  animal  dies  from 
suffocation,  or  asphyxia. 

Asphyxia  may  be  produced  in  several  ways.  It  may  be 
produced  by  stopping  the  windpipe,  either  from  within  by  a 
bolus  of  food  or  a  growth  within  the  larynx  or  trachea,  or  from 
without,  as  in  strangulation,  thus  preventing  ingress  of  oxygen  to 
the  lungs.  It  may  be  occasioned  by  the  lungs  being  filled  with 
an  inert  gas  or  an  inert  mixture  of  gases  containing  too  little 
oxygen,  as  in  the  after-damp  of  colliery  explosions,  or  in  the 
air  of  unventilated  cellars  or  sewers ;  or  by  the  lungs  being 
filled  with  water,  or  some  other  fluid,  as  in  drowning,  so  that 
the  oxygen  carmot  enter.  It  may  be  also  brought  about  by 
the  air  containing  a  much  smaller  quantity  of  a  poisonous  gas, 
such  as  carbon  monoxide,  which  forms  a  more  stable  compound 
with  haemoglobin  than  does  oxygen,  and  gradually  combines 
with  the  haemoglobin  permanently,  so  that  there  is  soon  not 
enough  left  to  act  as  an  efficient  oxygen  carrier  to  the  tissues.^ 
Finally,  it  may  be  caused  by  paralysis  of  the  nervous  mechanism, 
and  this  may  either  be  peripheral,  as  in  the  case  of  poisoning 
of  the  nerve  endings  of  the  respiratory  muscles  by  curare,  or 
central,  as  in  poisoning  by  morphia,  and  in  some  cases  of 
chloroform  administration  in  excess. 

When  the  supply  of  oxygen  is  insufficient,  there  is  an  ex- 
citation of  the  important  nerve-centres  lying  in  the  medulla 
oblongata  by  means  of  the  chemical  stimulus  of  the  too  venous 
blood  supplied  to  them. 

Thus,  the  cardio-accelerator  -centre  is  stimulated  and  the 
heart  beats  faster.  At  a  later  stage  the  cardio-inhibitory  centre 
is  excited,  and  the  heart  beats  slowly. 

^  Such  suffocation  occurs  in  poisoning  from  charcoal  fumes,  or  from 
coal  gas.  In  these  cases  the  carbon  monoxide  compound  has  a  cherry- 
red  colour,  which  gives  a  characteristic  hue  to  the  lips  and  complexion. 


Respiration,  1 89 

The  vaso-motor  centre  is  irritated,  and  nerve  impulses  are 
despatched  along  the  vaso-constrictor  fibres,  narrowing  the 
small  arterioles  generally  over  the  body,  and  thus  raising  the 
arterial  pressure. 

The  respiratory  centre  itself  is  affected,  and  there  is  at  first 
an  increase  in  both  the  number  and  depth  of  the  respirations, 
causing  laboured  breathing  or  dyspnoea. 

But  if  the  dearth  of  oxygen  continue,  the  venous  blood 
coming  to  these  centres,  which  at  first  stimulated  them,  later 
has  a  sedative  effect  upon  them ;  they  become  less  active,  and 
finally  they  are  paralyzed  and  cease  to  act. 

In  consequence,  the  efforts  at  respiration  become  slow  and 
gasping,  there  are,  later  on,  exaggerated  efforts  at  expiration 
only,  and  finally  all  respiratory  attempts  cease.  During  this 
period  the  vaso-motor  centre  also  passes  into  slumber,  the 
arterioles  relax,  and  the  arterial  pressure  falls.  The  heart- 
beats, which  had  become  slow  and  irregular,  in  part  from 
central  stimulation,  and  in  part  from  oxygen  starvation  of  the 
cardiac  muscle  fibres,  at  length  stop  altogether  from  the  latter 
cause.  The  arterial  blood  pressure  falls  to  zero,  the  circulation 
ceases,  and  the  animal  dies. 

Temporary  dearth  of  oxygen,  of  much  slighter  extent,  often 
occurs  during  the  life  of  an  animal.  We  see  this  evidenced  in 
the  laboured  breathing  which  follows  severe  muscular  exercise, 
and  in  the  freer  breathing  which  ensues  after  we  have  held  our 
breath  from  any  cause.  On  the  other  hand,  a  condition  can 
easily  be  induced,  known  as  apnoea,  in  which  the  animal  has  no 
desire  for  a  brief  time  to  breathe,  on  account  of  too  great  re- 
spiratory effort  immediately  preceding  it.^  Similarly,  during 
muscular  rest,  and  more  especially  during  sleep,  the  respiration 
is  quieter,  because  so  much  tissue  oxidation  is  not  going  on, 
and  hence  less  oxygen  is  required. 

An  apnoeic  condition  can  readily  be  produced  bv  taking  rapidiv  about 
a  dozen  deep  full  breaths,  when  a  short  pause  occurs, 'during  which 'there  is 
no  desire  to  breathe.  The  condition  is  chiefly  nervous  in  character,  for  it 
can  be  produced  by  distending  an  animal's  lungs  a  few  times  with  an 
inert  gas,  such  as  hydrogen.  It  is  said,  however,  that  apnoea  produced  by 
over-ventilation  with  oxygen  lasts  longer  than  that  similarly  produced  by 
an  inert  gas. 


IQO  Elementary  Physiology. 

The  increase  in  respiration  and  heart  action  following 
violent  muscular  action  is  one  of  the  best-known  phenomena  of 
our  lives.  The  demand  of  the  body  for  oxygen  is  suddenly 
very  much  increased,  as  well  as  the  necessity  for  increased 
excretion  of  carbon  dioxide,  and  there  are  obviously  two  means 
of  meeting  the  occasion.  First,  a  more  rapid  circulation, 
caused  by  increased  action  of  the  heart,  carrying  round  to  the 
tissues  a  larger  volume  of  oxygen,  and  removing  more  carbon 
dioxide.  At  the  same  time,  the  circulation  through  the  lungs 
is  also  made  more  rapid,  so  causing  an  increase  in  the  intake 
of  oxygen  and  in  the  output  of  carbon  dioxide.  Secondly, 
there  is  need  for  increased  ventilation  of  the  lungs,  to  throw 
out  the  excess  of  carbon  dioxide  discharged  there,  and  to  replace 
it  by  increased  supplies  of  oxygen  from  the  atmosphere ;  this 
is  provided  for  by  increased  respiration. 

These  desired  changes  in  the  rate  of  action  of  heart  and 
lungs  are  brought  about  by  stimulation  of  the  medullary  centres, 
by  the  character  of  the  blood  sent  to  them  by  the  heart ;  for 
this  soon  becomes  more  venous  as  the  muscles  go  on  working. 
There  is,  however,  a  maximum,  and  if  the  muscular  activity  be 
severe  and  prolonged,  the  changes  in  the  blood  begin  to  sub- 
stitute another  and  reverse  effect  instead  of  exaggerated  nervous 
activity ;  the  person  becomes  qidtc  out  of  breathy  or  completely 
pwnped  out^  and  has  to  slacken  off  or  desist  altogether. 

The  rate  of  respiratioji  is  slower  in  large  than  in  small 
animals.  One  reason  for  this  is  that  the  surface  of  the  skin  is 
larger  in  proportion  to  their  bulk  in  small  than  it  is  in  large 
animals,  and  hence  there  is  a  greater  comparative  loss  of  heat 
which  must  be  made  good  by  a  comparatively  greater  amount 
of  combustion,  and  hence  of  respiratory  exchange.  For  the 
same  reason  the  rate  of  the  heart-beat  is  quicker  in  small  than 
in  large  animals,  and  there  is  usually  a  fairly  constant  corre- 
spondence between  the  cardiac  and  respiratory  rhythms,  one 
respiration,  as  a  rule,  taking  place  for  about  four  heart-beats. 
For  the  same  reason,  heavy  persons  breathe  more  slowly  than 
light  persons,  and  the  adult  more  slowly  than  the  child.  At 
birth,  the  rate  of  respiration  is  usually  over  40  per  minute  ; 
at  five  years  of  age  it  lies  between  20  and  30 ;  in  middle  age 


Respiration.  191 

at  16  to  17;^  and  in  old  age  it  somewhat  increases  again, 
averaging  17  to  19  per  minute.  In  the  same  individual  con- 
siderable variations  in  the  rhythm  are  found,  according  to  the 
circumstances  under  which  observations  are  made.  The  effect 
of  muscular  exercise  has  already  been  alluded  to ;  posture  has 
also  a  great  effect,  the  rate  being  fastest  while  standing,  inter- 
mediate while  sitting,  and  slowest  while  lying.  The  rate  is 
also  affected  by  the  emotions  and  by  sensory  stimulation,  such 
as  application  of  cold  water  to  the  skin,  or  sudden  pain.  It 
can  only  temporarily  be  affected  by  direct  application  of  the 
power  of  the  will  to  that  object.  We  can  hold  our  breath 
voluntarily  for  a  short  time,  but  soon  the  desire  to  breathe 
becomes  imperative,  and  in  spite  of  the  utmost  voluntary  effort 
to  the  contrary,  respiration  recommences.  Also,  we  can  breathe 
faster  and  deeper,  or,  on  the  other  hand,  more  slowly  than 
normal  to  us,  for  a  short  interval  of  two  or  three  minutes ;  but 
it  is  impossible  to  keep  up  the  attempt  for  any  length  of  time, 
and,  in  spite  of  voluntary  effort,  we  soon  lapse  back  to  the 
normal  rate  and  strength  of  breathing. 

^  There    are    very   wide   variations    from    this    average,    any   number 
between  lo  and  24  per  minute  being  found  in  different  cases. 


CHAPTER    IX. 

ANIMAL  HEAT. 

All  animals  may  be  divided  into  two  great  classes,  according 
to  the  manner  in  which  they  react  to  changes  in  temperature  of 
their  surroundings.  In  one  class,  the  temperature  of  the 
animal's  body  does  not  vary  in  summer  or  winter,  but  remains 
at  a  constant  level,  provided  the  animal  is  in  a  healthy  con- 
dition ;  in  the  other  class,  the  temperature  of  the  body  does 
vary  with  that  of  the  surroundings,^  rising  when  the  air  or  water 
surrounding  the  animal  grows  warmer,  and  falling  when  the 
temperature  of  these  environments  sinks.  As  the  temperature 
of  the  first  class  of  animals  is  as  a  rule  both  higher  than  their 
surroundings  and  than  that  of  the  second  class  of  animals,  they 
are  termed  warm-blooded  animals ;  on  the  other  hand,  the 
animals  with  variable  body  temperature  are  termed  cold- 
blooded animals.^  To  the  former  class  belong  the  mammalia 
(including  man),  the  birds,  and,  to  a  certain  extent,  reptiles  ; 
to  the  latter,  amphibia,  fishes,  and  the  invertebrates. 

The  temperature  is  not  the  same  in  all  species  of  warm- 
blooded animals,  but  in  the  same  species  it  is  very  constant,  so 
much  so  tha.t  the  "  clinical  thermometer  "  becomes  an  invaluable 
test  for  a  feverish  condition  of  the  body,  because  the  tem- 
perature then  rises  above  normal.^ 

^  Although  it  is  not  identical  with  that  temperature,  but  somewhat 
higher. 

2  These  terms,  although  they  do  not  express  the  real  difference  between 
the  two  classes,  are  better  than  the  uncouth  terms,  homoiothermal  and 
poikilothermal,  which  have  been  proposed  instead  of  them. 

^  The  normal  temperature  in  man,  taken  in  the  axilla  or  arm-pit,  is 
37°  C.  (equal  to  98-6°  Fah.)  ;  in  the  mouth  it  is  slightly  higher,  37*2°  C, 
and  in  the  rectum  37 '6°  C.  The  blood  in  the  internal  parts  is  somewhat 
warmer  than  this,  and  is  hottest  in  the  hepatic  vein,  where  its  temperature  is 
about  39'5°  C. 


Animal  Heat,  193 

In  a  healthy  condition  of  a  warm-blooded  animal  the  means 
provided  in  the  body  for  regulating  the  temperature  are  so 
perfect  that  it  does  not  rise  or  fall  appreciably,  no  matter  how 
great  be  the  variation  in  the  temperature  of  the  surroundings, 
unless  the  exposure  be  very  great  and  prolonged.  "When  the 
temperature  does  vary  considerably  in  either  direction  the  con- 
dition of  the  animal  becomes  critical  and  life  soon  impossible. 

A  man  may  be  placed  in  the  rigour  of  a  polar  winter,  or 
beneath  the  burning  sun  of  the  tropics  in  summer,  but,  provided 
he  remain  in  a  healthy  condition,  the  temperature  of  his  body 
will  remain  the  same.  As  soon  as  the  adjusting  mechanism 
goes  out  of  order  this  constant  temperature  is,  how^ever,  no 
longer  retained,  and  the  temperature  of  the  body  may  go  above 
normal  and  remain  above  normal,  although  the  patient  be  sur- 
rounded with  ice-bags.^  A  person  in  a  healthy  condition  can  go 
into  a  very  hot  atmosphere,  such  as  that  of  a  Turkish  bath,  and 
remain  there  for  some  time,  although  the  temperature  may  be 
sufficient  to  cook  a  beef-steak,  but  such  a  person  must  be 
supplied  with  some  means  of  keeping  cool ;  a  large  amount  of 
water  must  be  drunk  to  supply  a  large  amount  which  is 
evaporated  from  the  skin  by  the  action  of  the  heat-regulating 
mechanism  of  the  body,  and  it  is  this  constant  evaporation  of 
water  w^hich  keeps  down  the  temperature  to  a  normal  level. 
Again,  a  person  who  is  exposed  to  a  low  temperature  must  be 
provided  with  means  of  producing  heat  to  replace  that  lost 
from  the  body  to  surrounding  objects,  otherwise  the  temperature 
of  the  body  would  fall.  X  liberal  supply  of  heat-producing  food 
must  therefore  be  eaten,  which  on  combustion  in  the  tissues 
yields  heat. 

Our  clothing  (and  similarly  the  wool,  feathers,  and  hair  of 
animals)  is  another  attempt,  apart  from  ornament,  to  aid  the 
heat-regulating  mechanism  of  the  body.  Clothes  and  other 
coverings  do  not  yield  heat  to  the  body,  they  merely,  when,  as 
is  usually  the  case,  the  temperature  of  surrounding  bodies  is 
lower  than  that  of  the  body,  prevent  loss  of  heat.     They  are 

^  In  such  a  condition  of  fever  the  combustion  in  the  tissues  becomes 
excessive,  and  the  regulating  mechanism  of  the  skin  is  unable  to  keep  pace 
with  it. 


194  Elementary  Physiology. 

bad  conductors  which  keep  in  the  heat  of  the  body,  and  so 
diminish  the  loss  by  conduction  and  radiation.  White  clothing 
reflects  a  good  deal  of  heat,  especially  direct  solar  heat,  and 
hence  is  cool  because  it  prevents  external  heat  from  entering ; 
on  the  other  hand,  black  clothing  absorbs  solar  heat  readily, 
and  hence  is  bad  clothing  for  hot  weather.  In  winter  we 
require  thick  clothing  of  material  which  does  not  conduct  heat 
well,  such  as  wool,  and  in  summer  light  clothing,  both  in 
texture  and  colour,  which  allows  the  heat  produced  in  the 
body  readily  to  escape,  and  also  reflects  as  much  as  possible 
the  external  heat,  and  does  not  allow  it  to  reach  the  body. 

It  is  by  being  able  in  this  way  to  modify  his  clothing  at  will, 
so  materially  aiding  the  natural  heat-regulating  agencies  of  his 
body,  that  man  is  enabled  to  inhabit  all  climates  of  the  globe, 
from  the  tropics  to  the  polar  regions. 

We  have  now  to  consider  the  ways  in  which  heat  is  pro- 
duced in  and  lost  from  the  body,  and  in  what  manner  the 
production  and  loss  are  so  balanced  as  to  produce  an  unvarying 
temperature. 

Heat  is  produced  in  the  body  by  chemical  change  (by 
oxidation  of  the  food),  and  the  heat  so  produced  is  distributed 
to  the  different  parts,  so  as  to  maintain  these  nearly  at  the 
same  temperature,  by  the  blood-stream,  which  in  this  respect 
acts  somewhat  like  a  hot- water  heating  apparatus.  The  blood 
is  heated  by  the  chemical  changes  going  on  in  the  glands 
(especially  in  the  liver)  and  in  the  muscles,  so  that  the  venous 
blood  passing  away  from  a  muscle  or  gland  is  warmer  than  the' 
arterial  blood  flowing  to  these  parts.  On  the  other  hand,  the 
blood  in  the  capillaries  of  the  skin  is  cooled,  to  some  extent 
by  radiation,  but  chiefly  by  evaporation,  on  the  surface  of  the 
skin,  of  the  water  separated  by  the  sweat  glands.  The  blood  is 
cooled  by  contact  with  the  cooler  skin,  and  therefore  the 
venous  blood  passing  away  from  the  skin  is  cooler  than  the 
arterial  blood  passing  to  the  skin. 

Another  way  in  which  the  body  loses  heat  is  by  evaporation 
in  the  air-passages  leading  to  the  lungs.  The  air  is  taken  into 
these  passages  in  a  more  or  less  dry  condition,  and  usually  at  a 
lower  temperature  than  that  of  the  body.     Some  heat  is  here 


Animal  Heat.  195 

lost  in  raising  the  air  to  the  temperature  of  the  body,  but  this  is 
inconsiderable  when  compared  with  the  larger  amount  which  is 
usually  lost  in  saturating  the  air  with  aqueous  vapour. 

It  takes  a  certain  definite  amount  of  heat  to  convert  a 
definite  weight  of  water  into  steam.  The  amount  of  heat 
which  so  becomes  latent  in  the  conversion  is  very  large,  for  it 
takes  nearly  six  times  as  much  heat  to  convert  boiHng  water 
into  steam  as  it  would  have  required  to  boil  the  same  quantity 
supposing  it  ice-cold  to  commence  with. 

An  amount  of  heat  practically  equal  to  this  becomes  latent 
whenever  water  becomes  changed  into  vapour,  no  matter  whether 
the  change  takes  place  at  the  temperature  of  boiling  water  or  not. 
Hence  the  amount  of  heat  lost  in  saturating  the  air,  which  is 
respired,  with  water  vapour,  and  in  evaporating  from  the  skin, 
at  the  temperature  of  the  air,  the  water  furnished  by  the  sweat 
glands,  is  very  large. 

The  production  of  heat  in  the  body  is  regulated  by  the  heat 
value  of  the  food  consumed,  and  by  the  amount  of  exercise 
taken  by  the  individual,  so  as  to  ensure  the  conversion  of  the 
whole  of  the  chemical  energy  of  the  food  into  heat  energy. 
The  .rates  at  which  exercise  is  taken  at  different  times  also 
regulate  the  rates  of  combustion  at  these  times.  \Vhen  an 
animal  is  exposed  to  cold  surroundings,  it  runs  or  walks  about, 
and  uses  its  muscles  so  as  to  keep  warm  by  the  heat  set  free. 
Also,  in  order  that  the  muscles  may  be  supplied  with  chemical 
energy  to  convert  into  heat  energy,  it  is  necessary,  if  the 
exercise  be  long  maintained,  that  the  food-supply  should  be  pro- 
portionately greater.  The  food  is  the  ultimate  source  of  the  heat ; 
the  muscular  energy  the  means  whereby  the  animal  is  enabled 
to  change  chemical  energy  into  heat  energy ;  and  the  blood- 
stream the  means  of  distribution  of  heat  energy  so  provided. 
On  the  other  hand,  when  an  animal  is  placed  in  warm  sur- 
roundings it  becomes  torpid,  there  is  no  need  for  great  heat 
production,  and  consequently  there  is  indisposition  to  muscular 
exercise,  so  as  to  produce  as  little  extra  heat  as  possible,  while 
at  the  same  time  the  loss  of  heat  is  increased  by  increasing  the 
blood-supply  to  the  skin.  The  appetite  also  becomes  less  keen, 
and  a  smaller  amount  of  food  suffices  for  the  wants  of  the  animal. 


196  Elementary  Physiology. 

The  amount  of  heat  lost  is  regulated  chiefly  by  the  blood- 
supply  to  the  vessels  of  the  skin.  When  we  are  exposed  to 
cold  surroundings  ^  the  skin  becomes  pale  and  bloodless ;  when 
we  are  exposed  to  a  warm  atmosphere  the  skin  becomes  flushed 
from  a  rich  supply  of  blood,  and  at  a  certain  limit  it  becomes 
wet  with  perspiration.  Before  this  limit  is  reached,  however, 
there  is  a  considerable  amount  of  water  being  evaporated  from 
the  skin,  only  the  evaporation  rate  exceeds  the  rate  at  which 
the  sweat  glands  pour  the  sweat  out,  so  that  there  is  no  accuma- 
lation  of  sweat  on  the  surface.  Even  when  the  air  in  contact 
with  the  skin  is  hotter  than  the  blood,  there  is  no  inconvenience 
felt  so  long  as  the  air  is  not  saturated  with  water  vapour,  and 
the  person  is  liberally  supplied  with  water.  But  if  the  hot  air 
is  also  saturated  with  vapour  it  soon  becomes  oppressive,  for 
then  the  sweat  is  not  evaporated  from  the  skin. 

To  sum  up,  then,  the  food  is  oxidized  in  the  tissues  by  the 
agency  of  the  oxygen  carried  by  the  haemoglobin  of  the  blood, 
and  (with  the  exception  of  a  small  fraction  which  is  turned 
into  external  work)  all  the  chemical  energy  so  set  free  is  changed 
into  heat.  This  supply  of  heat  keeps  the  animal's  body  at  a 
certain  uniform  temperature,  which  is  usually  above  (but 
exceptionally  may  be  below)  that  of  its  surroundings.  To 
maintain  this  constant  temperature,  heat  must  be  lost  at 
variable  rates,  according  to  the  changing  temperatures  of  the 
surroundings,  and  this  variation  is  accomplished  mainly  by 
varying  the  blood-supply  to  the  more  superficial  and  cooler 
part  of  the  body,  i.e.  the  skin,  and  by  varying  the  amount  of 
sweat  secretion  by  nervous  influence.^      Besides  losing  heat 

*  Such  an  exposure  produces  a  feeling  which  we  refer  to  as  cold,  and 
say  that  we  are  cold,  but  in  reality  the  body  does  not  become  any  colder 
unless  the  exposure  is  very  great,  and  then  numbness  and  unconsciousness 
supervene.  The  feeling  of  cold  is  a  warning  to  preserve  the  temperature  of 
the  body,  and  is  not  caused  by  any  appreciable  fall  in  temperature  of  the 
body  generally,  but  merely  of  the  skin.  The  same  is  true  of  the  feelings 
produced  by  warmth. 

^  It  is  supposed  that  there  are  specific  nerve  centres  for  heat  regulation, 
but  the  existence  of  these  can  scarcely  be  said  to  be  experimentally  proven. 
It  is  certain,  however,  that  vaso-motor  action  on  the  skin,  and  sweat 
secretion,  are  invoked  by  nervous  impulses,  which  may  originate  in  part 
from  the  change  in  temperature  of  the  skin,  affecting  peripheral  sensory  nerve- 
endings,  and  in  part  from  minute  changes  in  the  temperature  of  the  blood 
flowing  through  the  nerve-centres. 


Animal  Heat,  197 

through  the  skin,  the  body  loses  a  considerable  amount  by  the 
lungs,  which  is  chiefly  consumed  in  saturating  the  respired  air 
with  water  vapour. 

There  are,  besides,  other  minor  sources  of  loss  of  heat  to  the 
body,  which  are,  however,  of  no  great  importance  compared  to 
those  considered  above ;  such,  for  example,  as  the  food  being 
occasionally  taken  into  the  body  at  a  lower  temperature  than 
that  of  the  body,  while  the  excreta  are  voided  at  body 
temperature. 


CHAPTER   X. 

EXCRETION. 

The  waste  products  of  the  body  are  removed  by  four  channels  : 
viz.  by  the  kings,  in  the  expired  air ;  by  the  kidneys,  in  the 
urine  ;  by  the  skin,  in  the  sweat ;  and  by  the  alimentary  canal, 
in  the  faeces. 

There  is  daily  taken  in  along  with  the  food  a  certain 
amount  of  water,  and  an  equivalent  amount  of  water  to  this, 
together  with  a  much  smaller  amount  arising  from  the  oxidation 
of  the  hydrogen  of  the  food,  must  daily  be  removed  with  the 
proper  waste  products  of  the  body.  This  supply  of  water  is  as 
indispensable  to  the  animal  as  is  its  food;  it  may  be  taken 
mixed  with  the  food  as  water  of  the  food,  or  it  may  be  drunk 
alone,  but  in  some  form  it  must  be  taken  into  the  body.  We 
have  seen  that  the  food  is  absorbed  from  the  alimentary  canal 
in  solution,  and  water  is  essential  for  this  purpose.  Certain  of 
the  waste  products  are  removed  from  the  body  in  solution,  and 
here  again  water  is  necessary  as  a  vehicle  of  removal.  Further, 
a  considerable  amount  of  water  is  daily  evaporated  from  the 
skin  and  lungs,  and  it  is  chiefly  by  variations  in  the  amount  of 
water  so  removed  that  the  temperature  of  the  body  is  regulated 
and  kept  at  a  constant  level  in  spite  of  all  changes  in  the 
temperature  of  its  surroundings.  There  is  thus  a  constant 
stream  of  water  passed  through  the  body  which  carries  nutrient 
material  to  the  blood,  carries  waste  and  impurity  away  from  it, 
and  aids  in  regulating  the  body  temperature.  Although  this 
water  cannot,  strictly  speaking,  be  regarded  as  a  waste  product, 
yet  it  is  intimately  connected  with  the  removal  of  the  waste  pro- 
ducts, and  is,  moreover,  itself  removed  by  the  same  channels,  so 
that  it  can  be  conveniently  considered  along  with  them. 

A  very  little  of  the  waste  material  of  the  body,  and  a  very 


Excretion.  199 

small  proportion  of  the  water  are  removed  at  the  lo\Yer  end  of 
the  alimentary  canal  in  the  fseces.  Most  of  the  solids  of  the  faeces 
consist  of  waste  shreds  and  de'bris  of  the  food  which  have  never 
formed  part  of  the  body  and  are  merely  an  indigestible  residue 
of  the  food.  The  only  portion  which  can  be  regarded  as  a  true 
excretion  of  the  body  consists  of  a  small  amount  of  bile  pig- 
ments, cholesterin  and  lecithin  derived  from  the  bile,  and  a 
small  quantity  of  mucus  secreted  by  the  intestine  epithelial  cells 
and  serving  to  coat  the  mass  of  faeces  wdth  a  slimy  surface  to 
render  its  passage  easy.  The  w^ater  of  the  faeces  when  these 
are  in  a  normal  condition  forms  but  a  small  fraction  of  the 
quantity  of  water  daily  excreted ;  for  the  water  taken  in  with 
the  food,  together  with  that  added  by  the  secretions  which  are 
poured  in  at  the  upper  part  of  the  alimentary  canal,  is  rapidly 
removed  by  absorption  in  the  low^er  part  of  the  small  intestine 
and  in  the  large  intestine,  leaving  the  faeces  at  length  semi-solid 
in  consistency. 

Practically,  all  the  carbon  dioxide  excreted  from  the  body, 
as  well  as  a  fair  proportion  of  the  water,  is  removed  by  the 
lungs  in  the  expired  air.  The  manner  in  which  this  is  done 
has  already  been  considered  in  connection  with  respiration  j  it 
remains  to  describe  here  the  other  channels  of  removal — viz. 
the  skin  and  kidneys — and  the  constituents  which  they  remove. 

The  skin  chiefly  removes  w^ater,  accompanied  by  a  small 
amount  of  inorganic  salts  and  carbon  dioxide,  and  traces  of 
urea  and  other  nitrogenous  bodies. 

The  kidneys  also  remove  water,  but  their  chief  function  is 
the  removal  of  practically  the  "whole  of  the  nitrogen  formed  in 
the  degradation  of  proteid  in  the  body.  In  addition,  the  kidneys 
remove  the  chief  part  of  the  inorganic  salts  excreted  from  the 
body,  and  certain  salts  of  acids  of  the  aromatic  series,  which 
are  chemically  vei'y  stable^  and  being  formed  in  small  quantities 
(either  by  changes  going  on  in  the  body,  or  by  bacterial  action 
in  the  large  intestine  and  absorption  afterwards),  cannot  be 
broken  up  or  oxidized  in  the  body  subsequently,  and  hence  are 
excreted  unchanged  in  the  urine. ^ 

^  A  considerable  fraction  of  these  aromatic  compounds  is  excreted  as 
sulphates,  and  in  this  way  a   portion  of  the  sulphur  formed   in   proteid 


200  Elementary  Physiology. 

The  relative  amounts  of  water  excreted  respectively  by 
lungs,  skin,  and  kidneys  are  so  variable  with  varying  circum- 
stances that  no  average  figures  of  any  worth  can  be  given.  In 
cold  weather  the  relative  amount  excreted  by  the  kidneys  is 
increased,  for  then  the  blood-supply  to  the  skin  is  diminished 
and  the  production  of  sweat  is  decreased  because  loss  of  heat 
by  evaporation  is  not  so  much  required.  In  a  moist  condition 
of  the  atmosphere  the  amount  excreted  by  the  lungs  is 
diminished,  and  more  especially  if  the  air  be  both  warm  and 
moist;  for  the  amount  excreted  by  the  lungs  depends  on 
saturation  of  the  respired  air  with  water  vapour. 

The  Skin. 

The  skin  forms  a  protective  covering  for  the  surface  of 
the  body.  It  is  composed  of  two  parts,  termed  the  cutis  vera, 
corium  or  dermis,  and  the  epidermis  Or  scarf  skin  respectively. 
Of  these  two  the  cutis  vera  is  seated  more  deeply,  and  contains 
blood-vessels ;  while  the  epidermis  is  situated  superficially, 
forming  the  surface,  and  has  no  blood-vessels.  The  epidermis 
is  a  thick  stratified  epithelium  composed  of  a  large  number  of 
layers  of  cells,  which  get  different  names  at  different  depths,  as 
their  shape  and  consistency  alters  (see  Fig.  92). 

The  superficial  layers  are  flattened  and  horny,  those  nearest 
the  surface  being  quite  squamous,  while  the  deeper  layers  are 
somewhat  swollen.  In  these  cells  the  nuclei  have  degenerated 
or  at  least  become  invisible.  The  thin  pavement  cells  of  the 
outer  layers  are  gradually  worn  ofl'by  friction  and  abrasion,  and 
are  replaced  by  the  swollen  cells,  which  gradually  become  flat- 
tened as  they  near  the  surface,  while  in  turn  these  cells  are  re- 
placed by  others  from  deeper  layers.  The  deepest  stratum  of  the 
horny  layer,  lying  beneath  the  swollen  layer  above  mentioned, 
is  composed  of  clear  compressed  cells  and  is  known  as  the 
stratum  lucidum.  These  three  strata  of  the  horny  layer  merge 
by  easy  transition  into  each  other.  Beneath  the  horny  por- 
tion lies  the  softer  portion  of  the  epidermis,  which  is  known  as 

disintegration  is  got  rid  of.  The  balance  is  excreted  as  inorganic  sulphates. 
The  phosphorus  formed  in  the  breaking  down  of  proteid  appears  in  the 
urine  as  phosphates  of  the  alkalies  and  alkaline  earths. 


Excretion. 


20 1 


the  rctc  inucosum  (of  Malpighi) ;  in  this  part  the  nuclei  of  the 
cells  are  still  visible,  and  become  more  obvious  as  the  cells  are 
situated  more  deeply.  The  most  superficial  stratum  is  formed 
of  a  few  layers  of  granular  cells,  and  is  o-dW^^strattim gratiulosum  ; 


Fig.  92. — Section  of  epidermis.  (Ranvier.) 
H,  horny  layer,  consisting  of  5,  superficial  horny  scales  :  sw,  swollen-out  horny  cells ; 
S.I.,  stratum  lucidum  ;  IM,  rete  mucosum  or  !Malpighian  layer,  consisting  oi p, 
prickle-cells,  several  rows  deep  ;  c,  elongated  cells  forming  a  single  stratum  near  the 
cerium  ;  and  s.gr.,  stratum  granulosum  of  Langerhans,  just  below  the  stratum  luci- 
dum ;  n,  part  of  a  plexus  of  ner\-e  fibres  in  the  superficial  laj-er  of  the  cutis  vera. 
From  this  plexus  fine  varicose  ner\'e  fibres  may  be  traced  passing  up  between  the 
epithelium  cells  of  the  Malpighian  layer. 


beneath-  this  lies  a  thick  stratum  of  polygonal-shaped  cells 
known  as  prickle  cells  (of  Schulze),  which  have  small  inter- 
cellular channels  between  them  for  the  passage  of  lymph  for 
the  nutriment  of  the  cells.  The  channels  are  bridged  over  at 
intervals  by  processes  from  the  cells,  and  when  the  cells  are 
isolated  for  examination  under  the  microscope  these  processes 


202 


Elementary  Physiology. 


give  them  a  prickly  appearance,  from  whence  their  name  is 
derived.  In  the  deeper  layers  the  prickle  cells  tend  to 
become  columnar  in  shape,  with  the  longer  dimension  per- 
pendicular to  the  surface,  and  finally  there  is  a  layer  of  long 
columnar  cells  which  rests  upon  the  aUis  vera  lying  immediately 
beneath  it.^  The  under  surface  of  the  epidermis  does  not  form 
a  plane  surface,  but  is  thrown  into  ridges  and  hollows  by 
papillce  which  project  into  it  (see  Fig.  93) ;  these  papillae  bear 


Fig.  93. — Duct  of  a  sweat-gland  passing  through  the  epidermis.     (Magnified  200 
diameters.)     (Heitzmann.) 
BP,  papillae  with  blood-vessels  injected  ;   V,  rete  mucosum  between  the  papillae  ;  E, 
stratum  corneum  ;  PL,  stratum  granulosum  ;  D,  sweat-duct,  opening  on  the  surface 
at  P. 


the  blood-vessels  which  form  capillary  networks  in  the  super- 
ficial part  of  the  ctUis  vera  projecting  into  the  papillae.^  The 
outer  free  surface  of  the  epidermis  is  also  thrown  into  ridges 
corresponding  to  these,  but  not  so  deeply  marked,  which  form 
the  fine  markings  seen  on  the  finger  tips  and  elsewhere.  Thus 
no  blood-vessels  enter  the  epidermis,  but  its  deeper  layers,  where 

^  It  is  in  this  layer  that  the  pigment  is  developed  which  gives  colour  to 
the  skin  in  coloured  people, 

^  The  papillae  also  lodge  the  terminations  of  the  sensory  nerve  fibres 
for  tactile  sensation. 


Excretion.  203 

the  cells  are  growing,  are  fed  by  lymph  which  exudes  from  the 
capillary  networks  of  the  cutis  vera  and  can  pass  through  the 
intercellular  channels  above  mentioned.^ 

The  cutis  vera  is  composed  of  dense  connective  tissue, 
which  gradually  becomes  more  open  in  texture  as  it  passes 
into  the  subcutaneous  connective  tissue  which  underlies  it 
over  most  parts  of  the  body.  In  this  subcutaneous  connec- 
tive tissue  fat  may  be  largely  developed,  particularly  over  the 
abdomen,  where  it  forms  the  pamiiculiis  adiposus,  and  over 
the  buttocks.  The  cutis  vera  is  richly  supplied  with  blood- 
vessels, which  are  largely  distributed  to  the  surface,  forming 
the  capillary  networks  in  the  papillae  above  mentioned. 

The  skin  becomes  modified  at  various  parts,  as  at  the  lips, 
where  it  becomes  thin  and  transparent,  and  passes  gradually 
into  the  mucous  membrane  of  the  mouth;  and  on  the  inner 
surface  of  the  eyelids  and  over  the  eyeball,  where  it  is  changed 
into  a  thin  membrane,  called  the  conjunctiva.,  which  contains 
many  sensory  nerve-endings,  and  is  hence  extremely  sensitive. 
Besides  such  modified  parts  the  skin  has  modified  structures 
all  over  it,  which  are  known  collectively  as  the  appendages  of 
the  skiji ;  these  are  the  nails.,  the  haiis  and  their  glands 
(sebaceous  glands),  and  the  siueat  glands. 

The  nails  are  formed  by  thickening  and  alterations  of  the 
stratum  lucidum  in  certain  well-known  situations.  The  layers 
superficial  to  the  stratum  lucidum  disappear  in  the  course  of 
development  of  the  nail,  and  only  a  portion  remains  covered 
by  these  layers,  forming  the  narrow  band  at  the  root.  The 
nail  lies  on  the  nail-bed,  or  matrix,  which  is  formed  by  a 
modified  Malpighian  layer  with  longitudinal  ridges  and  grooves. 
The  nail  grows  forward  both  from  the  end  (nail  groove)  and 
from  the  posterior  portion  of  the  bed,  and  hence  the  free 
border  is  the  thickest  part  of  it.  The  substance  of  the  nail  is 
made  up  of  clear  compressed  horny  cells,  each  of  which  con- 
tains the  remains  of  a  nucleus. 

The  hairs  are  developed  in  the  hair  follicles  (see  Fig. 
95),  which  are  down-growths  of  the  epidermis  into   the  cutis 

'  On  the  other  hand,  fine  nerve  fibrils  do  pass  between  the  cells  of  the 
deeper  layers  of  the  mucosum  (see  Fig.  92). 


204 


Elementary  Physiology. 


vera,  or  even  into  the  subcutaneous  tissue  underlying  this. 
The  hair  grows  from  the  bottom  of  the  follicle,  where  it  is 
supplied  with  blood  by  a  small  vascular  papilla  which  projects 
up  into  the  somewhat  expanded  knob-like  end  of  the  hair. 
The  hair  substance  is  composed  of  a  pigmented  horny  fibrous 
substance  made  up  of  long  tapering  cells,  which  can  be 
separated  by  the  use  of  acids.     Externally  the  hair  is  covered 


Fig.  94.  —Section  across  the  nail  and  nail-bed.     (100  diameters.)     (Heitzmann.) 
P,  ridges  with  blood-vessels  ;  B,  rete  mucosum  ;  N,  nail. 


by  a  cuticle  of  imbricating  scales,  which  fit  against  similar 
scales  sloping  in  the  opposite  direction  on  the  inner  surface 
of  the  hair  follicle.^  The  central  part  of  the  hair  is  occupied 
by  a  dark-looking  material,  and  is  known  as  the  medulla.  When 
minute  air-bubbles  are  present  in  the  medulla,  or  between  the 

^  On  account  of  these  imbricating  scales  a  coarse  hair,  when  rubbed 
between  finger  and  thumb,  always  moves  towards  the  free  end,  just  as  does 
a  blade  of  coarse  grass  when  similarly  treated.  The  object  of  the  scales  is 
to  firmly  fix  the  hair  in  its  follicle. 


Excretion, 


20: 


fibrous  cells  of  the  hair,  the  hair  acquires  a  white  appearance 
seen  by  reflected  light. 
Each  hair  follicle  has 
one  or  more  small 
glands  in  connection 
with  it  which  are  known 
as  the  sebaceous  gla?ids 
(see  Fig.  95).  The 
ducts  of  these  glands 
open  into  the  hair  fol- 
licle near  its  mouth. 
The  glands  secrete  a 
fatty  material  called 
sebum,  which  is  pro- 
bably formed  by  the 
disintegration  of  the 
gland  cells.  The  sebum 
imparts  oiliness  and 
softness  to  the  hair. 
The  hair  follicle  has 
also  a  tiny  muscle  (ar- 
rector  pili)  attached  to 
it,  composed  of  in  volun- 
tary muscle  fibres.  This 
little  muscle  is  attached 
as  shown  in  the  figure, 
and  when  it  contracts 
it  erects  the  hair  or 
raises  it  at  right  angles 
to  the  surface  of  the 
skin. 

Sweat  glands  are 
seen  in  a  section  of  the 
skin  from  any  part  of 
the  body,  but  more 
abundantly  in  the  skin 
of  the  palm  or  sole, 
where     they    lie     very 


when 


a  o 


^  S 


Cm 


•5  ^ 


o  ..  t: 

'"S3 

Z.  'Z  o 

4,    C    <y 
-5    .      " 


^  "^  s~-r 


"=2  "? 

^   :-    ^ 
—   r:   i- 


206 


Elementary  Physiology 


close  together.  They  are  coiled  tubes  lined  with  cubical  or 
columnar  epithelium,  which  lie  deep  in  the  cutis  vera,  and 
send  their  ducts,  which  are  also  coiled  corkscrew  fashion,  up 
through  the  epidermis  to  open  on  the  surface.  The  glandular 
part  of  the  convoluted  tube,  in  which  the  sweat  is  secreted,  is 
lined  outside  by  a  basement  membrane  (see  Fig.  96),  inside 
which  is  a  single  layer  of  longitudinally  placed  fibres  which 


Fig.  q5. — Section  of  a  sweat  gland  in  the  skin  of  man. 
J!,  a,  secreting  tube  in  section  ;  b,  a  coil  seen  from  above  ;  c,  c,  efferent  tube  ;  d,  inter- 
tubular  connective  tissue  with  blood-vessels,     i,  basement  membrane  ;  2,  muscular 
fibres  cut  across  ;  3,  secreting  epithelium  of  tubule. 


resemble  involuntary  muscle  fibres,  and  more  internally  still 
surrounding  the  lumen  ^  of  the  tubule  there  is  a  single  layer 
of  columnar  cells  which  yield  the  secretion.  The  duct  leading 
from  the  secreting  portion  is  fined  by  two  or  three  layers  of 
cells,  and  the  lumen  is  much  narrower  (see  Fig.  96).  The 
duct  where  it  passes  through  the  epidermis  has  no  proper 
wall,  and  is  merely  an  excavated  channel  between  the  epithelial 
cells. 

'  The  lumen  is  the  central  opening,  or  bore,  of  a  gland,  alveolus,  duct, 
or  canal. 


Excretion.  207 

The  Sweat. — Sweat  is  continually  being  produced  by  the 
sweat  glands,  even  when  it  does  not  become  obvious  to  the  eye 
and  wet  the  skin.  It  only  accumulates  on  the  skin  when  it 
is  poured  out  by  the  glands  faster  than  it  can  be  evaporated 
off  by  the  air  in  contact  with  the  skin.  When  the  sweat 
accumulates  on  the  skin  the  condition  is  spoken  of  as  sensible 
perspiration  ;  while  the  term  inse7isible  perspiration  is  applied  to 
the  more  normal  case  where  it  is  produced  at  a  less  rapid  rate 
than  the  air  can  take  it  up,  so  that  it  does  not  appear  on  the 
skin.i  The  more  muscular  work  that  is  done  the  greater  will 
be  the  amount  of  heat  produced  in  a  given  time,  and  as  this 
heat  must  be  dissipated  from  the  body,  so  that  its  temperature 
may  remain  constant,  the  greater  will  be  the  amount  of  sweat 
produced  in  a  given  time  (see  p.  192);  and  hence  increased 
muscular  work  leads  to  sensible  perspiration.  Again,  the 
hotter  the  air  in  contact  with  the  skin,  the  more  water  must 
be  evaporated  from  the  skin  surface  to  keep  it  cool,  and  so 
cool  the  blood  circulating  underneath,  and  thus  keep  the  body 
temperature  normal ;  so  increased  temperature  of  surroundings 
leads  to  increased  sweat  production,  and  tends  to  sensible 
perspiration.  It  is  easily  seen  from  these  considerations  that 
the  rate  of  production  of  sweat  is  very  variable,  and  hence 
that  no  accurate  average  estimate  for  the  quantity  per  diem 
can  be  given,  but  it  probably  lies  between  one  and  three  litres.'^ 
Each  sweat  gland  is  supplied  by  a  small  arteriole  with  a  tuft 
of  capillaries,  and  the  blood-supply  to  these  is  controlled  by 
vaso-motor  nerve  fibres ;  there  are  also  secretory  nerve  fibres, 
through  w^hich  the  gland  cells  are  directly  stimulated  to  secrete. 
The  sweat  is  a  very  watery  secretion,  and  undoubtedly  its  chief 
function  is  to  regulate  the  body  temperature  by  water  evapora- 
tion from  the  surface,  and  not  to  purify  the  blood  by  excretion 
of  either  organic  or  inorganic  constituents.  It  contains  only 
from  o'5  to  2  per  thousand  of  total  solids,  of  which  about  one- 
third  is  inorganic  (chiefly  sodium  chloride),  and  the  remainder 

*  The  drier  the  air  the  more  rapidly  is  the  sweat  evaporated,  and  hence, 
the  temperatures  being  alike,  "sensible  perspiration"  is  more  easily  in- 
duced on  a  moist  day  than  on  a  dry  one. 

-  A  litre  is  1*76  pints. 


2o8 


Elementary  Physiology. 


organic.  It  has  an  acid  reaction,  probably  due  to  volatile  fatty 
acids;  but  after  profuse  sweating  the  reaction  may  become 
faintly  alkaline.  It  also  contains  traces  of  urea  and  of  carbon 
dioxide. 

The  Kidneys. 

The  urinary  system  consists  of  the  kidneys,  ureters,  bladder, 
and  urethra.     The  arrangement  of  these  parts  is  shown  in  the 


Fig.  97. — The  kidneys,  bladder,  and  their  vessels.     Viewed  from  behind. 

R,  right  kidney  ;  U,  ureter  ;  A,  aorta  ;  Ar,  right  renal  artery  ;  Ve,  vena  cava  inferior  ; 

Vr,  right  renal  vein  ;  Vu,  bladder  ;  Ua,  commencement  of  urethra. 

accompanying  figure  as  they  appear  from  the  back,  and  their 
situation  in  the  abdomen  has  already  been  described.  The 
urine  is  continuously  secreted  by  the  kidneys  and  trickles 
down  the  ureters  into  the  bladder,  where  it  accumulates,  until 


Excretion. 


209 


the  distension  of  the  bladder  gives  rise  to  a  feeling  of  uneasi- 
ness, when  it  is  voluntarily  discharged  through  the  urethra  by 
the  relaxation  of  a  sphincter  muscle  placed  at  the  commence- 
ment of  that  tube.  Thus  the  kidneys  secrete  the  urine,  and 
the  rest  of  the  system  is  an  accessory  part  for  its  temporary 
storage  and  convenient  expulsion.  Hence  the  kidneys  are  the 
most  important  portion,  and 
we  have  to  consider  here  their 
structure  and  the  nature  of 
their  secretion  or  excretion.^ 

When  a  kidney  is  split 
open  longitudinally  and  ex- 
amined, it  is  seen  to  present 
an  appearance  resembling 
that  shown  in  Fig.  98.  The 
ureter  enters  at  the  concave 
part,  called  the  hilum,  and 
expands  into  a  funnel-shaped 
dilatation,  which  is  termed  the 
pelvis.  The  pelvis  divides 
into  two  or  three  primary 
divisions,  and  these  again  sub- 
divide into  a  number  of  short 
wide  tubes,  which  are  named 
calices,  or  infundibula.  These 
receive  into  their  mouths  the 
ends  or  papillae  of  the  pyra- 
mids of  the  kidney  substance, 
and  are  attached  all  round  the 
bases  of  these  projections  (see 
Fig.    98),    so    as    to    receive 

and  carry  away  the  urine  which  issues  at  the  apices  of  the 
papillae. 

On  turning  the  attention  to  the  kidney  substance  it  can  be 

^  The  words  ''secrete  "  and  "  excrete  "  are  often  used  indiscriminately. 
Any  material  separated  by  a  glandular  structure  from  the  blood  may  be 
termed  a  secretion  ;  but  rigorously  a  secretion  means  a  fluid  material  service- 
able to  the  animal,  and  an  excretion  a  waste  product  separated  for  removal 
from  the  body. 

P 


Fig.  98.— Plan  of  a  longitudinal  section 
through  the  pehds  and  substance  of  the 
right  kidney.  One-half  the  natural 
size. 

a,  the  cortical  substance  ;  b.  b,  broad  part  of 
two  of  the  pyramids  of  Malpighi  ;  c,  c, 
the  divisions  of  the  pelvis  named  calices, 
or  infundibula.  laid  open  ;  c',  one  of  these 
unopened  ;  d,  d,  summit  of  the  pyramids 
or  papillae  projecting  into  calices  ;  e,  e, 
section  of  the  narrow  part  of  two  pyra- 
mids near  the  calices  ;  />,  pelvis  or  en- 
larged portion  of  the  ureter  within  the 
kidney  ;  j(,  the  ureter  ;  s,  the  sinus  ;  h, 
the  hilum. 


2IO 


Elementary  Physiology. 


seen  to  be  made  up  of  two  portions,  differing  in  appearance 
and  colour.     The  outer  portion,  lying  immediately  beneath  the 


Fig.  99. — Diagram  of  the  course  of  two  uriniferous  tubules.     (Klein.) 
A,  cortex  ;  B,  boundary  zone  ;  C,  papillary  zone  of  the  medulla  ;  a,  a',  superficial  and 
deep   layers  of  cortex,  free  from  glomeruli-     For  the  explanation  of  the  numerals, 
see  the  text. 


Excretion.  211 

fibrous  capsule  which  surrounds  the  kidney,  is  termed  the 
cortex.  It  is  nearly  uniform  in  its  appearance,  and  is  of  a 
reddish  brown  colour.  It  extends  inwards  to  the  bases  of  the 
conical  masses  known  as  the  pyramids  of  Malpighi^  and  also 
lies  between  contiguous  pyramids.  The  pyramids  constitute 
the  medulla  of  the  kidney;  they  vary  in  number  in  different 
animals ;  there  are  usually  over  twelve  in  the  human  kidney. 
The  substance  of  the  pyramids  is  distinctly  striated,  the  stri^ 
running  from  base  to  apex,  and  marking  the  course  of  the 
blood-vessels  and  uriniferous  tubules  {vide  infra),  which  here 
run  parallel  to  one  another  from  the  cortex  towards  the  papilla 
or  apex  of  the  pyramid.  The  urine  is  secreted  and  carried  to 
the  papilla  by  minute  tubules,  which  can  be  made  out  in  the 
kidney  substance  with  the  aid  of  a  dissecting  lens.  These 
tubules  are  termed  uriniferous  tubules,  and  by  very  patient 
dissection  under  a  lens  have  been  shown  to  have  a  very 
tortuous  course  in  the  kidney  substance,  which  is  represented 
diagrammatically  in  the  accompanying  figure. 

Each  tubule  begins  in  the  cortex  in  a  dil  ited  part  which  is  known  as  a 
Alalpighian  corpuscle.^  or  glomertcbis  (Fig.  99);  this  contains  a  much  con- 
voluted tuft  of  capillaries  over  which  the  tubule  commences  as  the  capsule. 
This  capsule  is  composed  of  a  double  layer  of  thin  pavement  cells,  one  of 
which  is  reflected  over  the  enclosed  capillaries,  while  the  other  forms  the 
outer  wall  of  the  glomerulus.^  Thus  the  blood  in  the  capillaries  is  separated 
from  the  uriniferous  tubule  only  by  the  thin  walls  of  the  capillaries  them- 
selves and  a  single  layer  of  flat  cells  forming  the  reflected  layer  of  the 
capsule.  The  outer  layer  of  the  capsule  narrows  to  a  neck  (2  in  Fig.)  at 
the  opposite  pole  of  the  glomerulus  to  that  at  which  the  blood-vessel  enters, 
and  the  cells  change  in  shape  from  pavement  to  cubical.  A  tube  is  thus 
formed,  which  in  the  first  part  of  its  course  is  convoluted  {first  convohited 
tiibiUe,  3  in  Fig.),  next  spiral  {spiral  tubide,  4  in  Fig.),  and  then  straight, 
running  down  the  medulla  in  one  of  the  pyramids.  The  tubule  next 
turns  back  towards  the  cortex,  forming  the  loop  of  Henle  (5,  6,  7,  8, 
and  9  in  Fig.),  and  in  the  cortex  again  becomes  zigzag,  and  then 
convoluted  {second  convohited  tuhile,  ii  in  Fig.),  finally  opening  by  a 
pcnctional  tubide  (i2  in  Fig.)  into  a  collecting  ticbide  (13  in  Fig.).  The 
collecting  tubule,  after  receiving  several  junctional  tubules,  passes  straight 

^  The  arrangement  is  as  if  the  capsule  forming  the  end  of  the  tubule  had 
been  a  ball,  against  which  the  tuft  of  capillaries  had  been  pushed  so  as  to 
force  in  the  ball  and  become  enclosed  in  such  a  way  that  one  layer  formed 
a  coat  for  the  capillary  tuft,  while  the  other  surrounded  the  whole. 


212 


Elementaiy  Physiology. 


down  the    medulla,   and    opens    at  the   apex 
Bellini  (15  in  Fig.)-    The  epithelium  lining 


Fig.  100.— Vascular  supply  of  kidney.     (Cadiat.) 
Diagrammatic. 

«,  part  of  arterial  arch  ;  b,  interlobular_  artery ;  c, 
glomerulus  ;  d,  efferent  vessel  passing  to  me- 
dulla as  false  arteria  recta  ;  e,  capillaries  of 
cortex  ;  /,  capillaries  of  medulla  ;  g,  venous 
arch  ;  h,  straight  veins  of  medulla  ;  j,  vena 
stellula  ;  i,  interlobular  vein. 


of  a  papilla  as  a  duct  of 
the  tubule  is  set  throughout 
on  a  basement  membrane, 
and  it  differs  in  character 
in  different  portions  of  the 
tubule.  In  the  first  convo- 
luted tubule  and  the  spiral 
tubule,  the  cells  are  cubical 
and  fibrillated,  and  the 
lumen  is  narrow.  In  the 
descending  limb  of  Henle's 
loop  and  the  loop  itself, 
the  cells  are  small  and 
flattened,  and  leave  a  larger 
lumen  ;  in  the  ascending 
limb  they  become  cubical 
again  ;  in  the  second  con- 
voluted tubule,  they  are 
cubical  and  fibrillated  ;  in 
the  junctional  tubule,  flat- 
tened ;  in  the  collecting 
tubule,  clear  and  cubical  ; 
and  in  the  duct  of  Bellini, 
clear  and  columnar. 

The  arrangement 
of  the  capillary  blood- 
vessels in  the  kidney  is 
somewhat  peculiar,  for 
after  being  gathered  up 
from  the  tuft  of  capil- 
laries in  the  glomerulus 
by  a  small  efferent 
vessel  which  leaves  the 
glomerulus,  the  blood 
is  again  distributed  to 
capillaries  by  the  sub- 
division of  this  efferent 
vessel,  and  the  second 
capillary  system  so 
formed  surrounds  the 
tubules  and  runs  along- 
side them. 


Excretion.  2 1 3 

Each  kidney  is  supplied  with  blood  by  a  single  large  artery  (renal  artery) 
arising  from  the  abdominal  aorta,  and  the  blood  after  circulating  through 
the  kidney  returns  to  the  inferior  vena  cava  by  a  large  vein  (renal  vein). 
These  blood-vessels  enter  at  the  hilum,  and  passing  into  the  kidney  substance 
form  a  system  of  large  branches  lying  between  cortex  and  medulla.  Smaller 
branches  are  given  off  from  these  (see  Fig.  lOO),  which  pass  to  cortex  and 
medulla  ;  those  which  go  to  the  cortex  supply  the  glomeruli,  and,  as  stated 
above,  again  form  capillaries  round  the  tubules  ;  on  the  other  hand,  those 
which  go  directly  to  the  medulla,  form  capillaries  only  once,  viz.  around 
the  medullary  portions  of  the  tubules. 

Urine  Secretion. 

The  glomerulus  looks,  by  reason  of  its  flattened  cells,  like 
a  simple  filtering  arrangement,  for  its  cells  do  not  appear  of  the 
proper  type  for  selective  secretion,  such  as  are  the  cells  lining 
other  parts  of  the  tubule.  Still,  the  pores  of  the  filtering  apparatus 
must  be  very  small,  and  unlike  those  of  porous  paper,  for  not 
a  trace  of  the  proteids  of  the  blood-plasma  comes  through  the 
glomeruli.  It  is  probable  that  certain  inorganic  salts  of  the  urine 
and  a  portion  of  the  water  pass  through  by  filtration  under 
pressure  at  the  glomeruli,  for  the  concentration  of  inorganic  salts 
in  urine  approximates  to  that  which  they  have  in  the  blood,  at 
any  rate  in  the  case  of  the  netifral  S2i\\.s,  such  as  sodium  chloride. 
But,  in  the  case  of  other  constituents,  it  is  obvious  that  the 
process  of  their  separation  is  not  one  of  simple  filtration  through 
the  glomeruli.  This  is  so  in  the  case  of  the  acid  inorganic 
salts  to  which  urine  owes  its  acidity  {vide  infra) ^  for  the  blood 
is  alkaline,  while  the  urine  is  normally  acid.  Still  more  so  is 
this  true  for  urea^  that  organic  constituent  which  is  present  in 
urine  in  largest  quantity,  and  is  of  most  importance. 

Urea  is  normally  present  in  urine  to  the  amount  of  2  per 
cent.,  while  in  blood-plasma  it  is  never  in  health  present  in 
more  than  y^o  of  this  concentration.  Hence  it  cannot  be 
present  in  the  urine  by  filtration  only,  but  must  be  actively 
secreted  by  some  of  the  cells  of  the  kidney.  The  kidneys 
also  rapidly  excrete  any  foreign  material  which  may  have  got 
into  the  blood,  such  as  drugs  and  medicaments,  and  these  are 
found  in  the  urine  in  much  stronger  solution  than  they  could 
ever  have  been  present  in  the  blood,  sure  evidence  that  they 
are  not  removed  by  filtration. 


214  Elementary  Physiology. 

Now  the  cubical  cells  lining  certain  parts  of  the  uriniferous 
tubules  possess  all  the  characteristics  of  secreting  cells,  and  it 
is  obvious  from  analogy  that  these  cells  have  some  secreting 
function,  as  well  from  the  appearance  of  the  cells  as  from  the 
convoluted  course  of  the  tubules,  and  the  abundant  blood- 
supply  to  their  cells.  Further,  there  is  a  certain  amount  of 
evidence  that  dissolved  foreign  substances  are  removed  from 
the  blood,  not  by  the  glomeruli  but  by  the  secreting  cells  of 
the  tubules.  Hence  it  is  probable  that  a  certain  portion  of 
the  water  and  some  of  the  inorganic  salts  are  separated  by 
filtration  in  the  glomeruli ;  while  other  inorganic  salts,  notably 
those  to  which  the  reaction  is  due,  the  urea  and  other  organic 
constituents,  as  well  as  any  foreign  substances  dissolved  by 
chance  in  the  blood,  are  removed  by  the  cells  of  the  tubules. 
The  secretion  of  the  cells  lining  the  tubule  is  thus  washed 
down  towards  the  collecting  tubules  by  the  watery  secretion, 
or  rather  filtration,  of  the  glomeruli. 

The  Urine. 

The  work  of  the  kidneys  is  to  regulate  the  condition  of 
the  blood,  and  to  keep  it  pure  by  removing  certain  waste 
products  formed  in  it  as  a  result  of  the  degradation  of  the  food. 
The  greater  part  of  the  carbon  dioxide  formed  in  the  body,  as 
already  stated,  is  removed  by  the  lungs,  and  a  certain  amount 
of  water  by  the  lungs  and  skin ;  all  the  balance  of  the  work  of 
maintaining  the  blood-stream  pure  is  done  by  the  kidneys. 

The  balance  between  kidney  activity  and  condition  of  the 
blood  is  a  very  delicate  one.  If  too  much  water  has  been 
absorbed,  so  that  the  blood  has  become  slightly  too  dilute,  the 
kidneys  immediately  commence  to  secrete  a  urine,  which  is 
much  poorer  in  solid  constituents  than  normal,  and  the  amount 
of  water  in  the  blood  is  rapidly  reduced.  If  the  blood  tends 
to  become  too  alkaline,  by  formation  from  it  of  an  acid 
secretion,  as  is  the  case  in  the  first  hours  after  a  meal,  then 
the  kidneys  at  once  commence  to  secrete  a  less  acid,  or  it  may 
be  even  an  alkaline,  urine  and  the  alkalinity  of  the  blood  is 
kept  unchanged  in  degree.  Suppose  some  substance  reaches 
the  blood-stream  which  is  an  abnormal  constituent  there,  then  it 


Excretion.  215 

is  at  once  treated  by  the  kidney  cells  as  an  enemy  and  promptly 
expelled.  Similarly,  an  excess  of  any  normal  constituent  of 
the  blood  is  treated  as  an  abnormality  and  excreted.  Blood 
normally  contains  about  two  parts  per  thousand  of  grape  sugar, 
and  so  long  as  the  concentration  does  not  rise  above  this 
the  kidney  cells  take  no  action  with  regard  to  it :  but  let  the 
quantity  increase,  and  at  once  the  cells  proceed  to  eliminate 
the  excess  in  the  urine.  In  diabetes,  the  sugar  found  in  the 
urine  is  not  formed  in  the  kidneys,  it  is  merely  thrown  out  by 
them.  Something  has  gone  wrong  elsewhere,  and  as  a  result 
there  is  an  excess  of  sugar  in  the  urine.  Thus,  the  presence  of 
sugar  in  the  urine  simply  shows  a  normal  attempt  on  the  part 
of  the  kidney  cells  to  remove  this  excess. 

Similarly,  the  presence  of  the  normal  constituents  of  urine 
in  that  excretion  is  due  to  the  preservation  of  a  balance; 
these  normal  constituents  are  being  continually  produced  in  or 
poured  into  the  blood-stream,  and  as  continually  are  they 
removed  by  the  kidneys,  so  that  each  is  kept  down  to  a  certain 
normal  percentage,  which  is  in  some  cases  very  low.  For 
example,  if  the  amount  of  proteid  in  the  food  be  increased,  so 
also  will  be  the  amount  of  the  products  of  proteid  waste  in  the 
blood ;  and  hence  the  amount  of  these  waste  products  excreted 
in  the  urine  will  be  correspondingly  increased. 

Chemical  Composition  of  Urine. 

Urine  is  a  clear  yellow  or  amber-coloured  fluid  which 
usually  has  an  acid  reaction.  The  acid  reaction  is  not  due  to 
free  acid,  but  to  acid  salts,  and  chiefly  to  the  acid  phosphate 
of  sodium  (NaH.POJ.  Th^  average  daily  amount  of  urine 
excreted  by  a  man  of  average  weight  {d^  kilograms,  or  145 
pounds)  is  1500  cubic  centimetres,  but,  as  already  mentioned, 
the  daily  amount  varies  very  considerably  with  the  amount  of 
fluid  drunk,  and  with  the  temperature  of  the  surroundings. 
The  total  amount  of  solids  in  this  quantity  of  urine  averages 
slightly  over  70  grammes,  of  which  about  40  grammes  is  organic 
and  30  grammes  inorganic  matter. 

The   chief    organic   constituents    of    the   urine   are   urea, 


2i6  Elementary  Physiology, 

creatinine,   uric   acid,   hippuric   acid,  pigment,    and  aromatic 
compounds  in  traces  usually  as  sulphates.^ 

The  inorganic  salts  are  chiefly  chlorides,  sulphates  and 
phosphates  of  sodmm^  potassium,  calcium,  magnesium,  and 
ammonium.^  Urea  is  the  most  important  of  the  organic 
constituents,  as  it  is  in  the  form  of  urea  that  nearly  all  the 
nitrogen,  of  the  proteid  of  the  food,  leaves  the  body.     The 

ATTT 

chemical  formula  of  urea  is  COCTxttt^;  and  it  hence  contains 

^  IN  J:l2 

nearly  half  its  weight  of  nitrogen. 

Urea  is  not  a  very  stable  body,  and  is  easily  oxidized  by  suitable  reagents 
to  carbon  dioxide,  nitrogen,  and  water.  Such  a  change  can  be  induced  by 
mixing  with  nitrous  acid  or  sodium  hypobromite,  and  may  be  represented 
by  the  following  equation  : — 

CO(NH2)2  +  sNaBrO  =  COg  +  No  +  2H2O  +  sNaBr 
or,  more  simply,  thus  : — 

C0(NH,)2  +  30  =  CO2  +  N2  +  2H2O 

This  reaction  is  taken  advantage  of  in  order  to  estimate  the  amount  of 
urea  in  any  given  sample  of  urine.  A  strongly  alkaline  solution  of  sodium 
hypobromite  is  used,  which  absorbs  the  carbon  dioxide  given  off,  and  only 
allows  the  nitrogen  to  escape.  From  the  volume  of  nitrogen  given  off,  the 
amount  of  urea  present  can  be  determined. 

After  urea  is  voided  in  the  urine  it  is  attacked  by  bacteria,  and  ammonium 
carbonate  is  formed.     The  chemical  action  is  one  of  hydrolysis  ;  thus — 

CO{NH2)2  +  2H2O  =  C03(NH,)2 

Creatinine  (C4H7N3O)  is  a  body  of  complex  chemical  constitution  which 
is  present  in  urine  in  small  quantity.  It  is  closely  related  to  creatine,  a 
disintegration  product  which  occurs  in  muscle,  especially  on  fatiguing  the 
muscle. 

Uric  acid  (C5H4N4O3)  is  a  bi-basic  acid  which  is  not  found  in  a  free 
condition  in  the  urine,  but  as  acid  sodium  and  potassium  urates.  It  is  not 
present  in  large  amount  in  mammalian  urine,  but  forms  the  greater  part  of 
the  solid  urine  of  birds  and  serpents.  The  free  acid  is  much  less  soluble 
than  its  salts,  and  is  hence  thrown  out  of  solution  on  standing,  after  adding 
a  strong  acid,  such  as  hydrochloric  acid,  to  urine.  Still,  the  urates  are  not 
very  soluble  in  water,  and  when  they  are  present  in  excessive  amount  they 


^  The  daily  amount  of  each  of  these  constituents  in  grammes,  in  round 
numbers,  is  as  follows  :  Urea,  33 '2  ;  creatinine,  9  ;  uric  acid,  5  ;  hippuric 
acid,  4 ;  pigment  and  other  organic  substances,  10  ;  chlorine,  7 '5  ;  sulphuric 
acid  (SO3),  2;  phosphoric  acid  (P2O5),  3;  sodium,  1 1  ;  potassium,  2*5  ; 
calcium,  magnesium,  and  ammonium,  i  '2. 


Excretion.  2 1 7 

pass  out  of  solution  ^vhen  the  urine  cools  as  a  brick-red  coloured  deposit, 
which  may  be  distinguished  from  other  urinary  deposits  by  the  fact  that  it 
disappears  on  warming  and  reappears  on  cooling. '  The  amount  of  urates 
in  the  urine  is  increased  by  excess  of  proteid  food,  and  by  sudden  excessive 
exercise. 

Hippuric  acid  (C9H9XO3)  is  a  compound  of  amido-acetic  acid  with 
benzoic  acid  ;  -  it  is  formed  chiefly  from  the  aromatic  compounds  present  in 
vegetable  food,  and  is  hence  present  in  much  greater  amovmt  in  the  urine 
of  herbivora.  Its  amount  in  the  urine  is  increased  by  administration  of 
benzoic  acid. 

The  pigments  of  the  urine,  like  those  of  the  bile,  are  probably  chiefly 
degradation  products  of  haemoglobin,  but  their  chemical  relationships  to 
that  substance  have  not  yet  been  definitely  made  out. 

*  Uric  acid  or  urates  may  be  further  recognized  by  the  red  colour  which 
they  give  when  evaporated  with  strong  nitric  acid,  turning  purple  on  making 
alkaline  with  ammonia  (murexide  test).  Uric  acid  is  insoluble  in  water, 
but  soluble  in  alkalies,  urates  being  formed  thereby. 

2  C6H5.COqH  +  CH2(XH2)COOH=:COOH.CH2(NH)COC6H5  +  H20 
Benzoic  acid.  Amido-acetic  acid.  Hippuric  acid. 


CHAPTER   XI. 

THE  NERVOUS  SYSTEM. 

The  nervous  system  controls  all  the  acts  of  the  life  of  the 
higher  animal.  It  determines  all  the  movements  of  the  body 
by  initiating  or  preventing  the  contraction  of  the  various 
muscles ;  it  determines  whether  or  not  the  cells  of  a  gland 
shall  be  passive  or  active,  and  so  whether  the  gland  shall  or  shall 
not  yield  a  secretion ;  it  regulates  by  its  action  on  the  arterioles 
the  supply  of  blood  to  the  various  parts,  according  to  their 
needs ;  it  gives  the  animal  information  as  to  its  surroundings 
by  conveying  impulses  from  the  outer  world  to  certain  struc- 
tures, themselves  belonging  to  the  nervous  system,  which 
are  capable  of  being  acted  upon  so  as  to  give  rise  to  what  are 
termed  sensations ;  it  further  carries  to  these  excitable  structures 
forming  part  of  itself  impulses  from  the  different  parts  of  the 
body  of  the  animal  which  give  information  as  to  the  situation, 
condition,  and  well  being  of  these  parts ;  finally,  it  is  an  organ 
capable  both  of  judging  and  estimating  present  sensations,  and 
of  retaining  impressions  of  past  sensations,  as  well  as  resolving 
its  judgment  on  this  complex  into  definite  acts  which  constitute 
the  life  of  the  animal  as  a  whole.  The  nervous  system  is  hence 
the  seat  of  intelligence,  and  the  more  complicated  and  highly 
developed  the  nervous  organization  is,  the  higher  is  the  grade 
of  intelligence  of  the  animal.  In  the  lowest  forms  of  animal 
life  no  definite  nervous  system  can  be  found.  It  first  appears 
in  the  form  of  isolated  knots  of  cells,  called  ganglia^  to  and 
from  which  long  processes  or  fibres  pass,  which  are  outgrowths 
of  the  nerve  cells.  Higher  in  the  scale  of  development  these 
ganglia  become  arranged  in  series,  or  chains,  along  the  axis  of 
the  body,  and  the  ganglia  are  connected  by  fibres  (nerve  fibres) 


The  Nervoits  System.  219 

passing  between  them.  In  vertebrate  animals,  the  series  or 
chain  of  ganglia  becomes  a  connected  whole,  in  which  nerve  cells 
are  present  along  the  entire  length,  and  bundles  of  fibres,  called 
nerves,  are  given  off  laterally  in  pairs  on  each  side.  The  nerve 
cells  and  the  fibres  connecting  them  together  become  enclosed 
in  a  bony  canal  running  along  the  vertebral  column,  from  which 
the  nerves  issue  between  the  successive  vertebrae ;  further,  at 
its  upper  end,  the  nervous  system  becomes  very  much  enlarged, 
forming  the  brain,  which  is  enclosed  in  the  bony  cavity  of  the 
skull  called  the  cranium.  The  extent  of  development  of  the 
brain  is  an  index  of  the  intelligence  of  the  animal  and  of 
its  position  amongst  the  vertebrates.  It  is  most  developed 
in  mammals,  most  of  all  in  man ;  and  amongst  various  races 
of  men,  the  most  highly  civilized  and  intellectual  have  also  the 
most  highly  developed  brains. 

The  nervous  system  is  usually  described  as  divided  into 
two  parts,  of  which  one  is  much  greater  than  the  other.  The 
part  enclosed  in  the  cranium  and  in  the  neural  canal  of  the 
vertebral  column,  with  the  nerves  attached  to  it,  is  spoken  of 
as  the  central  nervous  system,  or  cerebrospinal  system,  and  a 
double  chain  of  ganglia  situated  on  either  side  of  the  vertebral 
column  in  the  neck,  thorax,  and  upper  part  of  the  abdomen, 
and  connected  to  one  another  and  to  the  central  nervous 
system  by  nerve  fibres,  is  known  as  the  sympathetic  system. 
This  sympathetic  system  is  really  an  outgrowth  of  the  cerebro- 
spinal system.  The  part  of  the  nervous  system  lodged  in  the 
hollow  of  the  cranium,  and  in  the  neural  canal,  forms  the 
cerebro-spinal  axis,  consisting  of  the  brain  and  spinal  cord. 

By  far  the  greater  part  of  the  volume  of  the  brain  in  man 
and  in  the  higher  mammals  is  taken  up  by  the  cerebral  hemi- 
spheres which  occupy  the  upper  and  front  part  of  the  cranium 
(see  Figs.  loi  and  102).  The  two  hemispheres  are  separated 
from  each  other  by  a  deep  cleft  (the  longitudinal  fissure),  but 
are  united  to  each  other  at  the  bottom  of  this  fissure  by  a  thick 
band  of  nerve  fibres  (the  corpns  callosiim),  passing  from  the  one 
hemisphere  to  the  other  in  order  to  carry  nerve  impulses  across 
from  one  to  the  other.  The  surface  of  each  hemisphere  is 
marked  by  deep  groves  termed  stilci,  between  which  are  the 


220 


Elementary  Physiology, 


Fig.  ioi. — View  of  the  cerebro-spinal  axis.     (After  Eourgery.") 

The  right  half  of  the  cranium  and  trunk  of  the  body  has  been  removed  by  a  vertical 
section,  and  the  roots  of  all  the  spinal  nerves  of  the  right  side  have  been  dissected 
out  and  laid  separately  on  the  several  vertebrse  opposite  to  the  place  of  their  natural 
exit  from  the  cranio-spinal  cavity. 

F,  T,  O,  frontal,  temporal,  and  occipital  lobes  of  cerebrum ;  C,  cerebellum  ;  P,  pons 
Varolii ;  m  o,  medulla  oblongata  ;  ju  s,  in  s,  point  to  the  upper  and  lower  extremi- 
ties of  the  spinal  cord  ;  c  e,  on  the  last  lumbar  vertebral  spine,  marks  the  cauda 
equina  ;  C  i,  the  sub-occipital  or  first  cervical  nerve  ;  C  viii,  the  eighth  or  lowest 
cervical  nerve ;  D  i,  the  first  dorsal  nerve ;  D  xii,  the  last  dorsal ;  L  i,  the  first 
lumbar  nerve  ;  L  v,  the  last  lumbar  ;  S  i,  the  first  sacral  nerve  ;  S  v,  the  fifth  ;  Co  i, 
the  coccygeal  nerve ;  s,  the  left  sacral  plexus. 


The  Nei'vous  System. 


221 


eminences  known  as  the  convolutions.  The  purpose  of  this  is 
to  mcrease  the  surface  and  so  make  room  for  a  greater  number 
of  nerve  cells,  which  are  all  situated  in  a  thin  greyish  coloured 
layer  over  the  surface,  forming  what  is  called  the  cerebral  cortex. 
It  is  in  the  cells  of  this  cortical  layer  that  all  the  nerve  impulses 
which  start  from  the  cerebrum  originate.  The  deeper  part  of 
the  cerebral  substance  is  white  in  colour  and  contains  no  nerve 
cells,  but  only  nerve  fibres  passing  away  from  or  leading  to  the 


Fig.  I02. — Plan  in  outline  of  the  encephalon,  as  seen  from  the  right  side.     \ 

The  parts  are  represented  as  separated  from  one  another  somewhat  more  than  natural  so 
as  to  show  their  connections.  A,  cerebrum  ;  c,  fissure  of  Sylvius  ;  B,  cerebellum  ; 
C,  pons  Varolii  ;  D,  medulla  oblongata  ;  a,  peduncles  of  the  cerebrum ;  /',  t,  d, 
superior,  middle,  and  inferior  peduncles  of  the  cerebellum  ;  the  parts  marked  a,  b, 
form  the  isthmus  encephali. 

cells  of  the  cortex  and  serving  to  carry  impulses  to  or  from  these 
cells.  By  these  nerve  fibres,  the  cells  of  the  cerebral  cortex 
are  placed  in  communication  with  the  other  parts  of  the  central 
nervous  system  where  other  cells  are  situated.  Each  cerebral 
hemisphere  is  hollow,  and  the  small  cavity  inside  is  known  as 
the  ventricle.  On  the  floor  of  this  ventricle  are  situated  certain 
other  nerve  centres  containing  nerve  cells  to  which  some  of  the 
fibres  of  the  cerebral  cortex  run,  and  other  fibres  pass  from  these 
intermediate  centres  to  different  parts  of  the  brain.     By  far  the 


222 


Elementary  Physiology. 


greater  number  of  the  fibres  belonging  to  the  cerebral  cortex, 
however,  after  being  gathered  up  into  a  fan-shaped  bundle 
(known  as  the  corona  radiata)^  from  all  over  the  surface  of 
the  hemisphere,  unite  to  form  a  thick  stalk  or  bundle  of  fibres, 
which  passes  from  the  hemisphere  backwards  and  downwards 
towards  the  spinal  cord  and  is  known  as  the  crtis  cerebri  (see 
Fig.  102).    The  two  crura  cerebri  form  a  narrow  part  of  the  brain 


Fig.  103. — Right  half  of  the  brain  divided  by  a  vertical  antero-posterior  section  (from 
various  sources  and  from  nature.  (Allen  Thomson.) 
I,  2,  3,  3«,  3,5,  are  placed  on  convolutions  of  the  cerebrum  ;  4,  the  fifth  ventricle,  and 
above  It  the  divided  corpus  callosum  ;  5,  the  third  ventricle  ;  5',  pituitary  body  ;  6, 
corpora  quadrigemlna  and  pineal  gland  ;  +,  the  fourth  ventricle  ;  7,  pons  Varolii; 
8,  medulla  oblongata  ;  g,  cerebellum  ;  I,  the  olfactory  bulb  ;  11,  the  right  optic 
nerve  :  in,  right  third  nerve. 

which  is  known  as  the  isthmus  or  mid  brain.     In  the  mid  brain 

there  are  certain  other  intermediate  cell  stations,  or  nuclei,  of 

grey  matter,  containing  nerve  cells,  and  to  these  certain  of  the 

fibres  of  the  crura  cerebri  pass,  but  by  far  the  greater  number 

pass  downward  towards  the  spinal  cord.     Some  of  these  fibres 

pass  into  ^  the   cerebellum    and   form  the   superior  cerebellar 

peduncle. 

^  The  expression  "pass  into  "  is  here  used  in  an  anatomical  sense  ;  in  a 
physiological  sense  some  of  these  fibres  pass  into  the  cerebellum  and  carry 
impulses  to  it,  while  others  pass  out  and  carry  impulses  from  it. 


The  Nervous  System.  223 

The  cerebellum^  like  the  cerebrum,  consists  of  two  exactly 
similar  hemispheres,  which  are  also  deeply  indented  on  their 
smface,  giving  rise  to  convolutions,  but  not  in  an  exactly  similar 
manner  to  the  cerebrum  (see  Fig.  103). 

The  two  cerebellar  hemispheres  are  further  united  to  each 
other  by  a  thick  band  of  fibres  called  the  middle  cerebellar 
peduncle,  which  forms  part  of  the  brain  known  as  the  pons 
Varolii}  A  third  peduncle,  called  the  inferior  peduncle, 
connects  each  cerebellar  hemisphere  with  the  spinal  cord  below. 
The  cerebellum  is  thus  connected  both  with  the  parts  of  the 
brain  above  and  below  and  with  its  fellow  of  the  opposite  side. 
It  has,  like  the  cerebrum,  a  cortex  which  contains  nerve  cells, 
and  the  white  part  underlying  this  cortex  contains  only  nerve 
fibres. 

A  great  number  of  the  nerve  fibres  of  the  crura  cerebri 
do  not  pass  off  laterally  to  the  cerebellum,  but  pass  down 
underneath  the  crossing  fibres  of  the  middle  cerebellar 
peduncle  to  reach  the  cord  (see  Fig.  106) ;  these  are  joined  in 
this  course  by  the  fibres  of  the  inferior  cerebellar  peduncle,  and 
together  these  bundles  of  fibres  make  up  the  greater  part  of  the 
medulla  oblongata^  which  is  the  name  given  to  that  part  of  the 
brain  intervening  between  the  pons  Varolii  and  the  spinal  cord. 
In  the  medulla  there  are  imbedded  certain  masses  of  grey 
matter  containing  nerve  cells,  and  here  several  pairs  of  nerves 
take  origin  ;  also  at  this  part,  the  white  and  grey  matter  begin 
to  change  their  relative  situation  and  tend  to  assume  that  which 
they  occupy  throughout  the  spinal  cord.  In  fact,  the  medulla 
may  be  regarded  as  the  part  where  transition  from  brain  to  cord 
takes  place.  We  have  seen  that  in  the  hemispheres  of  the  brain 
the  nerve  cells  lying  in  the  grey  matter  occupy  the  cortex  or 
external  part ;  while  the  nerve  fibres  conducting  the  impulses 
from  these  nerve  cells  lie  more  internally,  forming  the  white 
matter.  In  the  spinal  cord  the  position  is  reversed,  for  the 
cells  in  the  grey  matter  occupy  the  central  part  of  the  cord  and 
are  everywhere  surrounded  by  nerve  fibres  forming  the  white 

^  A  band  of  fibres  so  uniting  two  bilaterally  similar  parts  of  the  central 
nervous  system  is  known  as  dL  commissure ;  thus  the  corpus  callosiim  is  a  great 
commissure,  so  is  the  middle  cerebellar  peduncle,  and  there  are  similar 
commissures  uniting  the  grey  masses  in  the  spinal  cord. 


224  Elementary  Physiology, 

part  of  the  cord.  These  nerve  fibres  run  parallel  to  each  other 
down  the  length  of  the  cord,  in  long  strands.  As  they  pass 
down  the  cord  their  number,  and  hence  the  volume  of  the  white 
matter,  decreases ;  for  they  gradually  pass  in  as  they  go  to  com- 
municate with  the  nerve  cells  at  different  levels  in  the  cord. 
The  purpose  of  these  fibres  is  to  give  communication  between 
the  cerebrum  and  other  parts  of  the  brain  and  the  nerve  centres 
of  the  spinal  cord,  as  well  as  intercommunication  between  the 
different  parts  of  the  cord  itself. 

At  the  lower  and  anterior  part  of  the  medulla  oblongata  a 
large  number  of  fibres,  coming  from  the  cerebral  cortex,  cross 
over  to  the  opposite  side  and  decussate  in  bundles  as  they  cross 
from  each  side,  thus  forming  what  is  called  the  decussation  of  the 
pyramids  (see  Fig.  to6)  ;  for  these  fibres  form  part  of  what  is 
known  as  the  pyramidal  tract.  The  pyramidal  tract  commences 
in  the  nerve  cells  of  a  portion  of  the  cerebral  cortex  surrounding 
a  transverse  fissure  in  the  brain  called  the  fissure  of  Rolando 
(near  A,  Fig.  102).  This  region  of  the  cerebral  cortex  is  known 
as  the  motor  area,  because  the  nerve  cells  of  this  area  regulate 
voluntary  motion,  and  when  certain  portions  of  the  area  are 
injured,  voluntary  motion,  in  definitely  corresponding  parts  of 
the  body,  is  paralyzed.^  From  the  motor  area  of  the  cortex  the 
pyramidal  fibres  pass  down  in  the  corona  radiata  to  the  crus 
cerebri  of  the  same  side,  and  so  reach  the  medulla,  where  at  the 
decussation  about  four-fifths  of  them  cross,  but  the  proportion  is  ' 
very  variable.  In  the  cord  there  are  thus  two  pyramidal  tracts 
on  each  side — that  which  has  come  from  the  opposite  side  of 
the  brain  occupies  the  postero-lateral  part  of  the  white  matter 
of  the  cord,  and  is  known  as  the  crossed  pyramidal  tract;  while 
that  which  has  not  crossed  at  the  decussation  lies  in  the  anterior 
part  of  the  white  matter,  and  is  termed  the  dii-ect  pyramidal 
tract.  These  fibres  pass  eventually  to  motor  nerve  cells  ^  lying 
at  different  levels  in  the  cord,  and  hence  the  tracts  decrease  in 

^  Also,  on  stimulating  different  parts  of  this  area  electrically,  move- 
ments of  different  parts  take  place  ;  and  so  well  localized  are  the  areas  for 
different  movements,  that  it  can  be  predicted  with  precision  what  movement 
will  follow  stimulation  of  a  definite  point  of  the  motor  cortex  (see  Fig.  112), 

^  These  cells  lie  in  the  anterior  horn  of  the  grey  matter  of  the  cord 
{pide  infra). 


The  Nervous  System. 


225 


volume  as  the  cord  is  descended.  Although  the  fibres  of  the 
direct  pyramidal  tract  do  not  cross  at  the  decussation,  they  do 
all  cross  at  various  levels  lower  down  in  the  cord,  and  are  all 
eventually  connected  with  nerve  cells  of  the  opposite  side  of 


Fig.  104.— Different  views  of  a  portion  of  the  spinal  cord  from  the  cervical  region  with 

the  roots  of  the  nerves.  Slightly  enlarged.  (Allen  Thomson.) 
In  A,  the  anterior  or  ventral  surface  of  the  specimen  is  shown,  the  anterior  nerve  root  of 
the  right  side  having  been  divided  ;  in  B,  a  view  of  the  right  side  is  given  ;  in  C, 
the  upper  surface  is  shown  ;  in  D,  the  nerve  roots  and  ganglion  are  shown  from 
below.  I,  the  anterior  median  fissure  ;  2,  posterior  median  fissure  ;  3,  antero-lateral 
impression,  over  which  the  bundles  of  the  anterior  nerve  root  are  seento  spread 
(this  impression  is  too  distinct  in  the  figure)  ;  4,  postero-lateral  groove  into  which 
the  bundles  of  the  posterior  root  are  seen  to  sink  ;  5,  anterior  root ;  5',  in  A,  the 
anterior  root  divided  and  turned  upwards  ;  6,  the  posterior  root,  the  fibres  of  which 
pass  into  the  ganglion,  6';  7,  the  united  or  compound  nerve;  7',^  the  posterior 
primary  branch,  seen  in  A  and  D  to  be  derived  in  part  from  the  anterior  and  in  part 
from  the  posterior  root. 

the  cord.  It  follows  from  this  complete  crossing  of  the  motor 
fibres  that  an  injury  to  the  motor  area  of  the  brain  will  cause 
motor  paralysis,  not  on  the  same  side  of  the  body  as  the 
injury,  but  on  the  opposite  side. 

When  the  spinal  cord  is  cut  across,  or  when  thin  sections  of 
it  are  made  and  examined  with  the  microscope,  the  situation  of 
the  grey  and  white  matter,  and  the  structure  of  each,  can  be 

Q 


226 


Elementary  Physiology. 


made  out.  The  grey  matter  is  arranged  in  two  somewhat 
comma-shaped  masses,  connected  by  commissures  passing  in 
front  of  and  behind  a  small  central  foramen,  which  is  the  central 
canal  of  the  spinal  cord  (see  Fig.  105). 

The  thicker  end  of  each  comma  is  called  the  anterior  horn, 
or  cornu,  and  contains  many  large  conspicuous  nei*ve  cells. 


Fig.  105. — Section  of  spinal  cord  in  the  lower  cervical  region. 

which,  are  connected  with  motor  nerve  fibres  passing  to  the 
peripheral  parts  of  the  body.  The  more  pointed  posterior 
part  is  called  the  posterior  horn  or  cornu,  and  to  this  the 
sensory  fibres  pass  which  convey  sensory  impulses  from  the 
periphery  (skin,  etc.)  towards  the  central  nervous  system. 

The  spinal  nerves  arise  in  pairs  from  the  cord  at  intervals, 
which  correspond  roughly  to  the  vertebrae,  and  the  nerves  issue 
between  the  vertebrae.  Each  nerve  arises  by  two  roots  (see 
Fig.  104)  from  the  cord,  which  are  termed  the  anterior  and 
posterior  roots.  On  the  posterior  root  a  ganglion  is  situated, 
containing  the  nerve  cells  belonging  to  the  fibres  of  that  root, 
and  all  these  fibres  are  sensory  or  affe^'ent^  that  is,  they  carry  nerve 

^  "  Afferent  "  means  leading  impulses  to  the  central  nervous  system,  and 
"efferent  "  leading  impulses  from  the  central  nervous  system  :  these  are  more 
general  terms  than  "  sensory  "  and  "motor  ;  "  for  all  fibres  leading  nerve 
impulses  to  the  cord  are  not  sensory,  and  all  leading  from  are  not  motor. 


The  Nervons  System.  227 

impulses  inwards  from  the  periphery  to  the  posterior  horn  of  the 
grey  matter.  The  anterior  root  has  no  gangHon,  and  the  nerve 
cells  belonging  to  the  fibres  contained  in  this  root  are  situated 
in  the  anterior  horn  of  the  grey  matter.  All  the  fibres  of  the 
anterior  root  are  motor  or  efferent^  and  convey  nerve  impulses 
from  the  central  nervous  system  (anterior  cornu)  to  the  periphery. 

The  union  of  the  two  roots  gives  rise  to  the  mixed  nerve 
trunk,  in  which  some  of  the  fibres  are  afterent  and  some 
efferent.  These  facts  as  to  the  nature  of  the  fibres  in  the  two 
spinal  nerve  roots  have  been  made  out  from  the  effects  of 
section  or  of  stimulation  of  each.  When  all  the  posterior  roots 
of  the  nerves  going  to  a  limb  are  cut,  sensation  is  lost  in  the 
limb,  but  it  can  still  be  moved.  When,  on  the  other  hand,  all 
the  anterior  roots  are  cut,  sensation  is  still  present  in  the  limb, 
but  it  cannot  be  moved.  Again,  when  the  central  end  of  the 
divided  anterior  root  is  stimulated  electrically  no  effect  is 
obtained ;  but  when  the  peripheral  end  is  so  stimulated  there  is 
a  movement  of  the  limb.  Further,  when  a  similar  experiment 
is  made  with  the  posterior  root  no  effect  is  obtained  now  on 
stimulating  the  peripheral  end  ;  but  with  the  central  end  there  is 
evidence  that  an  impulse  has  reached  the  cord  in  the  move- 
ments of  the  opposite  limb,  or  of  all  the  other  limbs  of  the 
body,  according  to  the  strength  of  the  stimulus,  or  of  the  same 
limb  if  the  anterior  roots  passing  to  it  have  not  been  divided. 

The  spinal  cord  does  not  extend  quite  to  the  end  of  the 
neural  canal,  but  passes  at  the  lower  end  of  the  body  of  the  first 
lumbar  vertebra  into  a  slender  filament  called  thejihwi  termi7iale. 
The  lower  lumbar  nerve  roots  pass  downward,  to  their  exit, 
parallel  to  the  filum  terminale,  forming  together  what  is  known 
as  the  Cauda  eqtnnce. 

Besides  the  spinal  nerves  there  are  twelve  other  pairs  of 
nerves  which  arise  from  the  cerebro-spinal  axis.  These  nerves 
leave  the  brain  within  the  cranium,  and  are  hence  called  the 
cranial  nerves.  Enumerated  from  before  backward  the  cranial 
nerves  are  as  follows  : — 

I.  The  olfactory  nerves  arise  from  the  olfactory  tract,  which 
is  an  outgrowth  from  the  frontal  lobe  of  the  cerebrum.     About 

^  See  note  on  p.  226. 


228  Elementary  Physiology. 

a  dozen  filaments  pass  from  the  olfactory  tract  to  the  mucous 
membrane  (olfactory  mucous  membrane)  lining  the  upper  part 
of  the  nasal  passages. 

II.  The  optic  nerves  passing  backward,  one  from  each  eye, 
meet  in  the  middle  line  to  form  the  optic  commissure,  from 
which  the  two  optic  tracts  pass  outward  and  backward  (see 
Fig.  1 06)  to  be  distributed  to  those  intermediate  cell  stations, 
already  mentioned  (see  p.  221),  in  the  floor  of  the  ventricle 
of  the  cerebrum  and  in  the  mid  brain.  These  intermediate 
stations  are  connected  by  other  fibres  with  the  posterior 
part  of  the  cortex  of  the  cerebrum  (occipital  lobe),  which  is 
chiefly  concerned  with  visual  sensations  (see  Fig.  112,  p.  242). 

III.  The  oculo-motor  nerves  supply  some  of  the  muscles  of 
the  eyeball  and  of  the  interior  of  the  eye. 

IV.  The  trochlear  nerves  supply  one  of  the  muscles  of  the 
eyeball  (superior  oblique). 

V.  The  t?'igei?imal  nerves  are  so  called  because  they  divide 
into  three  chief  branches  (see  V,  i,  2,  and  3,  Fig.  106)  :  viz.  {a) 
the  ophthalmic  division,  which  chiefly  supplies  sensory  fibres  to 
the  eyeball,  to  the  interior  of  the  eye,  and  to  neighbouring  parts ; 
ip)  the  superior  maxillary  division,  which  supplies  sensory  fibres 
to  the  skin  of  the  temple  and  cheek,  to  the  teeth  of  the  upper 
jaw  and  surrounding  parts;  and  (^)  the  inferior  maxillary  division, 
which  supplies  sensory  fibres  to  the  mucous  membrane  on  the 
inner  surface  of  the  cheek,  to  the  anterior  two-thirds  of  the 
tongue,  and  the  floor  of  the  mouth,  to  the  teeth  of  the  lower 
jaw,  to  the  chin,  lower  lip  and  margin  of  the  jaw,  and  to  the 
muscles  of  mastication.  This  division  also  supplies  motor  fibres 
to  the  muscles  of  mastication. 

VI.  The  abdncejit  nerves  are  the  smallest  of  the  cranial 
nerves,  and  are  merely  motor  nerves  in  each  case  to  that  muscle 
which  causes  outward  movement  of  the  eyeball  (external  rectus). 

VII.  The /^i;(r/<^/ nerves  supply  motor  fibres  to  the  muscles 
of  the  face,  and  secretory  fibres  to  the  salivary  glands. 

VIII.  The  auditory  nerves  supply  the  organs  of  hearing ; 
they  also  send  fibres  to  peculiar  structures  in  the  internal  ear, 
called  the  semi-circular  canals,  which  are  concerned  with  the 
sensation  of  equilibrium. 


The  Nervotis  System, 


229 


Fig.  106. — View  from  before  of  the  medulla  oblongata,  pons  Varolii,  crura  cerebri,  and 
other  central  portions  of  the  encephalon.     (Allen  Thomson.)     Natural  size. 

t',  the  olfactory  tract  cut  short  and  lying  in  its  groove  ;  II,  the  left  optic  nerve  in  front 
of  the  commissure  ;  II',  the  right  optic  tract  :  Th,  the  cut  surface  of  the  left  thala- 
mus opticus  ;  G,  the  central  lobe  or  island  of  Reil  ;  Sy,  fissure  of  Sylvius  ;  X  X, 
anterior  perforated  space  ;  e,  the  external,  and  i,  the  internal  corpus  geniculatum  ; 
h,  the  hypophysis  cerebri  or  pituitary  body ;  tc,  tuber  cinereum  with  the  infundibu- 
lum  ;  a,  one  of  the  corpora  albicantia;  P,  the  cerebral  peduncle  or  crus  ;  III,  close 
to  the  left  oculo-motor  nerve  ;  X,  the  posterior  perforated  space.  PV,  pons  Varolii ; 
V^,  the  greater  root  of  the  fifth  nerve  ;  +,  the  lesser  or  motor  root  ;  \'l,  the  sixth 
nerve  ;  VII,  the  facial  ;  VIII,  the  auditory  nerve  ;  IX,  the  glossopharyngeal ;  X, 
the  pneumogastric  nerve  ;  XI,  the  spinal  accessory  nerve  ;  XII,  the  hypo-glossal 
nerve  ;  C  I,  the  suboccipital  or  first  cerA'ical  nerve  ;  p  a,  p3"ramid ;  o,  olive  ;  d,  an- 
terior median  fissure  of  the  spinal  cord,  above  which  the  decussation  of  the  pjTamids 
is  represented  ;  c  a,  anterior  column  of  cord  ;  r,  lateral  tract  of  bulb  continuous 
with  c  I,  the  lateral  column  of  the  spinal  cord. 


230  Elementary  Physiology, 

IX.  The  glossopharyngeal  nerves  are  the  nerves  of  taste  to 
the  posterior  third  of  the  tongue ;  they  are  also  sensory  nerves 
for  this  region  and  the  neighbouring  parts,  as  well  as  the  upper 
part  of  the  pharynx. 

X.  The  pneiimogastric'^  nerves  have  a  very  important  and 
wide  distribution.  From  the  wandering  course  of  the  nerve  it 
is  also  called  the  vagus.  It  contains  both  afferent  and  efferent 
fibres,  and  sends  branches  to  the  pharynx,  larynx,  oesophagus, 
heart,  lungs,  stomach,  intestine,  pancreas,  and  liver.  By 
means  of  these  branches  it  exercises  an  important  control 
on  the  operations  of  swallowing,  digestion,  and  secretion,  and 
also  plays  a  part  in  regulating  the  rhythm  of  respiration  and  of 
the  heart-beat. 

It  contains  both  afferent  and  efferent  fibres ;  among  the  afferent  fibres 
are  those  to  the  respiratory  centre,  which  differ  in  their  action  according  to 
the  part  of  the  distribution  of  the  nerve  from  which  they  come.  When  both 
vagal  trunks  are  cut  in  the  neck,  the  respiration  becomes  very  slow,  and  if 
now  the  central  end  of  one  vagus  be  stimulated  the  respiratory  rhythm  is 
greatly  quickened,  showing  that  the  vagus  trunk  contains  afferent  fibres,^ 
which  normally  have  an  accelerating  effect  on  the  rhythm  of  respiration. 
On  the  other  hand,  if  those  fibres  of  the  vagus  contained  in  its  superior 
laryngeal  branch,  which  supply  the  mucous  membrane  of  the  larynx,  be 
stimulated,  respiration  is  inhibited,  inspiration  is  stopped,  and  a  violent 
expiratory  effort  takes  place.  Such  a  stimulation  naturally  takes  place 
when  a  particle  of  food  or  other  foreign  matter  comes  in  contact  with  the 
mucous  membrane  of  the  larynx,  and  the  purpose  of  the  expiratory  effort  so 
produced  is  to  expel  the  particle  and  prevent  it  from  dropping  into  the 
trachea.  Examples  of  the  distribution  of  the  efferent  fibres  of  the  vagus 
are  :  the  motor  fibres  to  the  muscles  of  the  larynx  in  the  inferior  laryngeal 
branch;  the  secretory  fibres  to  the  glands  of  the  mucous  membrane  of 
stomach  and  intestine,  and  to  the  pancreas  which  stimulate  the  cells  of  these 
glands  and  cause  them  to  secrete  during  digestion  ;  the  cardio-inhibitory 
fibres  to  the  heart,  which  slow  the  rhythm  of  that  organ  or  stop  it  temporarily 
if  stimulated  sufficiently  strongly.^ 


^  So  called  because  it  gives  off  branches  to  both  lungs  and  stomach. 

^  These  afferent  fibres  produce  their  effect  reflexly  (see  p.  237)  :  when 
stimuli  passing  along  them  reach  the  respiratory  centre  situated  in  the 
medulla  oblongata,  reflex  motor  stimuli  are  sent  out  from  this  centre,  which 
travel  along  the  motor  nerves  to  the  respiratory  muscles  and  cause  these 
muscles  to  contract. 

^  Such  stimuli  constantly  reach  the  heart  along  the  vagi  during  life,  as 
is  shown  by  the  great  quickening  in  the  heart-beat  which  always  follows  the 
cutting  of  both  vagi.  That  the  vagi  here  act  as  efferent  nerves  carrying 
impulses  to  the  heart  is  shown  by  the  fact  that  electrical  stimulation  of  the 
peripheral  end  of  the  cut  nerve  {i.e.  the  end  next  the  heart)  causes  slowing, 
or  if  strong  enough  temporary  stoppage  of  the  heart. 


The  Nervous  System.  231 

XI.  The  spinal  accessoi'y  nerves  arise  by  two  different  roots, 
one  in  the  medulla  and  the  other  in  the  lower  cervical  part  of 
the  spinal  cord.  This  nerve  gives  branches  to  the  vagus,  and 
it  has  been  shown  that  it  is  to  fibres  derived  from  the  medullary 
origin  of  the  spinal  accessory,  and  which  join  the  vagus,  that 
the  cardio-inhibitory  action  of  the  vagus  is  due.  The  fibres 
which  arise  from  the  cervical  spinal  cord  chiefly  supply  motor 
fibres  to  certain  of  the  muscles  of  the  neck. 

XII.  The  hypoglossal  nerves  take  origin  from  the  medulla 
oblongata,  and  are  motor  nerves  which  supply  the  muscles  of 
the  tongue. 

The  sympathetic  nervous  system  consisting,  as  has  been 
already  mentioned,  of  a  double  chain  of  ganglia  {i.e.  knots  of 
nerve  cells)  connected  to  each  other  by  nerve  fibres,  is  an 
outgrowth  of  the  central  nervous  system,  and  is  connected  to 
it  at  intervals  along  its  length  by  strands  of  nerve  fibres  {i-ami 
coinimmicantes) .  The  fibres  of  the  sympathetic  system  act 
chiefly  on  the  vascular  and  visceral  system,  sending  branches 
to  the  secreting  glands,  which  control  the  character  of  their 
secretion  j  to  the  walls  of  the  small  arteries  {v as o-motor  fibres)^ 
controlling  their  calibre ;  to  the  heart,  quickening  its  rhythm, 
and  so  antagonizing  the  vagus ;  to  the  intestinal  muscular  walls, 
controlling  their  peristalsis ;  to  the  iris  of  the  eye,  increasing 
the  diameter  of  the  pupil  (see  p.  269);  and,  generally,  the 
sympathetic  fibres  may  be  said  to  carry  out  operations  necessary 
to  the  well-being  of  the  animal,  but  outside  the  control  of  its 
will.  On  the  other  hand,  the  nerve  impulses  of  the  central 
nervous  system  are  partially  voluntary  and  partially  involuntary. 

We  may  next  briefly  consider  the  minute  structure  of  the 
nerve  cells,  nerve  fibres,  and  nerve  endings  which  form  the 
essential  parts  of  the  nervous  mechanism. 

If  one  of  the  peripheral  nerve  trunks  be  exposed,  such  as 
the  sciatic  nerve  in  the  thigh,  and  a  small  length  be  cut  out 
and  teased  with  needles  in  normal  saline  solution,  it  will  be 
found  that  the  nerve  is  not  easily  broken  across,  but  that  it 
divides  rather  easily  into  strands  along  its  length.  If  now  a 
minute  strand  be  taken  and  teased  out  in  saline  solution 
and  then  examined  under  the  microscope,   it  will  be  found 


Elementary  Physiology. 


c^  .a 


<U       dj 


-13 


•^  o 


D    <U 


/'i  J;/  1 


The  Nervous  System,  233 

that  the  nerve  is  made  up  of  bundles  of  long  fibres  which  run 
parallel  to  one  another  in  the  length  of  the  nerve  without 
branching.  The  nerve  fibres  are  bound  compactly  together  by 
connective  tissue  which  envelopes  the  whole  nerve  trunk  in  a 
sheath  from  which  septa  are  sent  in  dividing  the  nerve  trunk 
into  bundles.  The  nerve  fibres  do  not  branch  or  communicate 
with  one  another  throughout  the  whole  length  of  their  course. 
By  appropriate  straining  and  examination  they  may  be  shown 
to  have  the  structure  represented  in  the  accompanying  illus- 
trations (Figs.  107,  108). 

At  intervals  there  are  constrictions  of  the  fibre,  which  are 
termed  the  nodes  of  Ranvier.  At  the  nodes  the  outer  thin 
sheath  of  the  fibre,  known  as  the  neurilemma^  is  constricted, 
and  the  inner  coat  {medidiary  s/ieath),  which  is  thick  and 
consists  of  soft  fatty  material,  is  wanting.  Here,  where  the 
medullary  sheath  is  discontinuous,  there  may  be  made  out  a 
thin  central  fibre  passing  without  interruption  through  the  node. 
This  thin  fibre  is  termed  the  axis  cylinder ;  when  the  medullary 
sheath  is  dissolved  away  by  treatment  with  ether,  it  may  be 
made  out  passing  continuously  from  node  to  node  throughout 
the  length  of  the  fibre. 

The  axis  cylinder  is  the  essential  part  of  the  nerve  fibre, 
and  the  other  coats  are  protecting  sheaths  for  it.  It  begins  as 
a  process  of  a  nerve  cell,  and  pursues  an  uninterrupted  course 
from  the  nerve  cell  to  the  nerve  termination  throughout  the 
entire  length  of  the  nerve  fibre.  That  the  neurilemma  or 
primitive  sheath  and  the  medullary  sheath  are  accessory  parts 
only,  is  shown  by  the  fact  that  each  is  absent  in  certain  situa- 
tions, while  the  axis  cylinder  is  never  wanting.  Thus,  in  the 
spinal  cord  and  brain,  where  the  nerve  fibre  is  protected  by 
other  means,  and  where  individual  nerve  fibres  never  run  alone 
as  they  do  in  the  terminal  branchings  of  the  peripheral  ner^^es, 
the  outer  sheath  is  not  required,  and  hence  is  absent.  Again, 
in  the  sympathetic  system,  the  medullary  sheath  is  but  feebly 
developed,  and  is  often  absent,  giving  rise  to  what  are  termed 

^  Also  called  the  primitive  sheath  or  sheath  of  Schiuatiii ;  beneath  this 
sheath  cell  nuclei  are  placed  at  intervals.  These  nuclei  are  best  shown  by 
straining  with  hasmatoxylin  after  treating  with  acetic  acid. 


234 


Elementary  Physiology. 


noii-medullated  fibres  (see  Fig.  io8),  the  usual  variety  of  nerve 
fibre  being  called  medulla  fed  fih-e. 

The   nerve  cells   from  which  the  nerve  fibres  arise  vary- 
greatly  in  appearance  in  different  parts  of  the  nervous  system. 


Fig.  log. — Multipolar  nerve  cell  from  anterior  horn  of  spinal  cord,  human.     (Gerlach.)' 
a,  axis-cylinder  or  nerve-fibre  process  ;  b,  pigment. 

Some  are  very  large  with  a  large  clear  spheroidal  nucleus  con- 
taining a  nucleolus,  while  others  are  very  small.  They  are 
classified,  according  to  the  number  of  processes  which  they 


The  Nervous  System.  235 

give  off,  into  unipolar,  bipolar,  and  multipolar.  By  far  the 
larger  number  are  multipolar,  as  this  is  the  chief  type  found 
in  the  brain  and  spinal  cord ;  ^  there  are  in  this  type  a  large 
number  of  processes,  one  of  which  becomes  an  axis  cylinder  of 
a  nerve  fibre,  while  the  others  branch  into  fine  fibrils  termed 
dendrons  (see  Fig.  109).  Bipolar  and  unipolar  nerve  cells  are 
chiefly  found  in  the  ganglia  of  the  posterior  roots  of  the  spinal 
nerves.  The  nerve  fibre  of  one  nerve  cell  never  communicates 
directly  with  another  nerve  cell,  but  breaks  up  into  branches 
which  interlace  with  the  dendrons  of  that  cell.  The  fibres  of 
peripheral  nerves  communicate  with  their  nerve  cell  at  the 
central  end ;  in  the  case  of  the  afferent  fibres  this  nerve  cell 
is  situated  in  the  ganglion  of  the  posterior  root;  in  the  case 
of  the  efferent  fibres  it  is  placed  in  the  anterior  cornu  of  the 
grey  matter  of  the  spinal  cord.  At  the  periphery  these  fibres 
terminate,  either  in  ramifications  formed  by  subdivision  called 
plexuses,  or  in  special  end  organs.  Plexiform  terminations  are 
found  in  the  cornea,  in  involuntary  muscle  and  in  certain  glands. 
Special  end  organs  are  found  in  voluntary  muscle,  tendon,  and 
skin  ;  some  of  these  are  shown  in  the  diagrams  (Figs,  no,  in). 
In  certain  cases,  such  as  the  tactile  corpuscles  in  the  skin, 
a  nerve  impulse  originates  in  these  end  organs,  and  travelling 
towards  the  central  nervous  system  gives  rise  to  a  sensation 
when  it  arrives  there ;  ^  and  in  other  cases  a  nerve  impulse 
arriving  at  the  end  organ  from  the  central  nervous  system 
starts  cell  activity  there.  For  example,  on  arriving  at  a 
muscular  end  plate  (see  Fig.  in)  a  nerve  impulse  gives 
rise  to  contraction  of  the  muscle  fibre.  The  life  of  a  nerve 
fibre  depends  upon  its  connection  with  the  nerve  cell  to  which 
it  belongs ;  when  severed  from  this  the  fibre  undergoes  certain 
changes  which  are  spoken  of  as  nerve  degeneration.  Only  the 
part  of  the  fibre  which  is  cut  off  from  the  nerve  cell  undergoes 
these  changes,  the  part  still  in  connection  remains  unchanged 

'  The  cells  here  are  further  classified  according  to  their  shape,  e.g.  the 
pyramidal  cells  of  the  cerebral  cortex,  and  the  pear-shaped  cells  of  Purkinje 
in  the  cerebellum. 

^  The  specialized  nerve  terminations  in  the  organs  of  special  sense,  such 
as  the  eye  and  ear  (see  p.  245),  may  be  regarded  as  such  end  organs  in  which 
nerve  impulses  arise  giving  origin  in  the  brain  to  special  sensations. 


236 


Elementary  Physiology. 


in  its  structure.  This  gives  an  important  method  of  investi- 
gating the  connections  and  course  of  nerve  fibres,  especially  in 
the  brain  and  cord.  By  its  means  tracts  have  been  discovered 
in  the  spinal  cord,  for  the  fibres  of  the  white  matter  do  not  run 
indiscriminately,  but  those  with  nerve  cells  situated  above  lie 
in  one  part,  and  those  with  nerve  cells  below  lie  in  a  different 
part.     Hence  some  tracts  of  the  white  matter  degenerate  below 


Fig.  iio. — Tactile  corpuscle  within  a 
papilla  of  the  skin  of  the  hand, 
stained  with  chloride  of  gold. 
(Ranvier.^ 

Hi  two  nerve  fibres  passing  to  the  cor- 
puscle ;  a,  a,  varicose  ramifications 
of  the  axis  cylinders  within  the 
corpuscle. 


FiG._  III.  —  Nerve-ending 
in  muscular  fibre  of  a 
lizard  (Lacerta  viri- 
dis).    (Kiihne.) 


{descending  tracts)^  after  section  at  any  level,  while  other  tracts 
degenerate  above  {ascending  tracts).  In  this  manner  the 
ascending  and  descending  tracts  shown  in  Fig.  105  have  been 
discovered. 

The  nerve  fibres  of  the  sympathetic  system  have  their 
nerve  cells  in  the  ganglia  of  the  sympathetic  chain,  and  with 
these  nerve  cells  fibres  from  the  spinal  cord  communicate. 

In  general  terms,  then,  the  nervous  system  may  be  spoken 
of  as  an  exceedingly  complex  meshwork  of  nerve  cells,  nerve 


The  New 07 IS  System.  2'^'j 

fibres,  and  nerve  endings,  in  close  communication  with  one 
another,  and  capable  of  affecting  one  another,  and  calling  one 
another  into  activity.  By  communicating  fibres  all  the  different 
parts  of  the  brain  and  spinal  cord  are,  as  it  were,  put  in  touch 
with  one  another;  and  there  are  usually  several  paths  of 
nervous  communication  between  any  two  given  points,  so  that 
when  one  is  broken  down  from  any  cause  another  may  be 
used.  At  different  points,  nerves  containing  immense  numbers 
or  fibres  pass  off  to  all  the  peripheral  parts  to  supply 
communication  between  the  central  nervous  system  and  all 
the  other  parts  of  the  body ;  some  of  these  carry  messages 
from  the  centre  (efferent  fibres),  while  others  carry  messages 
to  the  centre  (afferent  fibres).  In  addition,  there  are  special 
nerves,  capable  of  being  rendered  active  by  light,  sound,  etc., 
which  convey  impressions  to  the  brain  of  what  is  going  on 
in  the  outside  world,  and  give  rise  to  actions  in  the  nervous 
system,  which  in  turn  are  resolved  into  actions  upon  the  other 
tissues. 

The  nerve  cells  are  not  arranged  indiscriminately  in  the 
brain  and  cord,  but  those  cells  which  carry  out  a  common 
purpose  are  placed  close  together.  A  collection  of  nerve  cells 
which  possess  a  common  purpose  is  termed  a  nerve  centre. 
Such  nerve  centres  are  found  in  the  different  parts  of  the  cord, 
medulla  oblongata,  and  brain.  The  centres  in  the  cord  preside 
over  the  muscular  movements,  and  are  capable  of  carrying 
out  quite  complicated  movements  without  assistance  from  the 
brain.  This  is  especially  the  case  in  lower  types  of  vertebrates, 
such  as  the  frog;  in  the  mammalia  generally,  and  in  man, 
these  spinal  centres  are  much  less  independent,  and  are 
completely  under  the  control  of  the  more  highly  developed 
brain. 

The  simplest  form  of  a  complete  nervous  action  is  what 
is  termed  a  reflex  act.  In  this  an  afferent  impulse  starting  at 
the  termination  of  an  afferent  nerve  fibre  passes  up  to  the 
nerve  centre  and  affects  a  nerve  cell  there ;  this  in  turn  aftects 
another  nerve  cell  connected  with  an  efferent  fibre,  and  a 
nerve  impulse  travels  down  the  efferent  fibre,  and  shows 
itself  by  some  action  in  the  tissue  to  which  this  efferent  fibre 


238  Elementary  Physiology, 

passes.  For  example,  irritation  of  the  skin,  by  pricking  with 
a  sharp  instrument,  or  by  contact  with  a  drop  of  acid,  may 
stimulate  sensory  nerve  endings  in  the  skin,  thus  starting  nerve 
impulses,  in  the  afferent  fibres  attached  to  these  nerve  endings, 
which  reach  the  nerve  centre  in  the  spinal  cord ;  these  next 
stimulate  motor  nerve  cells  and  start  efferent  impulses  down  the 
fibres  belonging  to  these  cells  which  reach  motor  end  plates 
in  voluntary  muscles  and  cause  contraction  and  muscular 
movements. 

Such  reflex  movements  may  be  carried  out  by  the  nerve 
centres  in  the  spinal  cord  without  at  all  affecting  the  brain, 
and  indeed  can  take  place  after  the  brain  has  been  removed, 
or  after  the  cord  has  been  cut  across  and  thus  removed  from 
the  influence  of  the  brain.  Injuries  to  the  spinal  cord  which 
practically  amount  to  separation  from  the  brain  are  often 
observed  in  man  as  the  result  of  accident  or  disease,  and  it 
is  then  seen  that  although  the  muscles  of  the  part  of  the  body 
below  the  injury  are  no  longer  under  the  control  of  the  will, 
they  can  be  moved  when  the  skin  of  the  part  is  irritated. 
Such  movements  are  reflex  in  character,  and  the  centre  for 
the  reflex  lies  in  the  spinal  cord ;  the  brain  is  not  affected  by 
them,  and  the  patient  is  not  conscious  either  of  the  irritation 
of  the  skin  or  of  the  muscular  movements,  except  by  seeing 
them.  Suppose,  for  example,  the  spinal  cord  is  injured  in 
the  dorsal  region  so  as  to  be  no  longer  capable  of  transmitting 
nerve  impulses  up  or  down ;  then  the  legs  become  paralyzed, 
the  patient  can  neither  move  them  voluntarily,  nor  can  he  feel 
anything  in  them.  If  now  the  sole  of  the  foot  be  tickled  the 
leg  will  be  drawn  up  violently,  but  the  patient  declares  that 
the  tickling  was  not  felt,  and  that  he  made  no  effort  to  draw 
up  his  leg — in  fact,  he  is  only  conscious  that  the  leg  moves  by 
seeing  it,  or  by  the  movement  disturbing  other  parts  of  his 
body  from  which  his  brain  can  still  receive  sensory  impulses. 

There  is  not  much  co-ordination  ^  in  these  purely  spinal 
reflexes  in  man;  they  are  quite  disorderly,  and  in  the  case 
of  other  mammals,  although  the  co-ordination,  on  recovery 
after  complete  division  of  the  cord,  is  greater,  it  is  still  by  no 

^  See  p.  73. 


The  Nervous  System.  239 

means  perfect.  It  is  in  cold-blooded  animals,  such  as  the 
frog,  that  the  cord  is  most  perfect  as  a  reflex  centre.  When 
the  head  of  a  frog  is  cut  off,  or  its  brain  destroyed  by  pithing, 
the  remainder  of  the  animal  is  still  capable  of  carrying  out 
most  involved  and  complicated  co-ordinated  movements.  If 
it  be  hung  from  a  support,  and  a  small  piece  of  paper  moistened 
with  acid  be  applied  to  one  flank,  or  to  the  abdomen,  there 
is,  after  a  pause,  a  movement  of  the  leg  obviously  intended  to 
stroke  the  irritated  spot.  If  the  leg  of  the  same  side  be  held 
and  prevented  from  carrying  out  this  purpose,  then  after  a 
longer  pause,  there  is  a  movement  of  the  opposite  leg,  likewise 
designed  to  remove  the  source  of  irritation.  If  the  piece 
of  paper  be  applied  to  the  back  of  the  thigh,  the  muscular 
movement  is  of  quite  a  different  type,  but  designed  also  to 
brush  the  irritated  spot.  The  movements  are  so  purposeful 
as  to  suggest  that  they  are  directed  by  an  intelligent  reasoning 
cause,  and  there  is  no  doubt  that  the  nerve  centres  in  the  cord 
of  such  an  animal  act  as  subsidiary  brains,  and  that  although 
the  frog  as  an  individual  is  dead,  yet  there  remains  a  nervous 
mechanism  to  all  tests  as  intelligent  as  the  entire  nervous 
system  of  an  animal  of  a  somewhat  lower  type.  Such  an 
amount  of  independent  action  is  not  found  in  the  spinal  cord 
in  warm-blooded  animals,  where  these  centres  become  more 
subsidiary  to  the  chief  centres  in  the  brain.  In  warm-blooded 
animals  the  chief  centre  for  voluntary  co-ordination  of  com- 
plicated movements  lies  in  the  cerebellum,  and  when  this  is 
removed  or  injured  by  disease  it  is  found  that  co-ordination 
becomes  exceedingly  faulty. 

The  functions  of  the  different  parts  of  the  brain,  in  so  far  as 
they  are  yet  known  to  us,  have  been  made  out  in  part  from  the 
study  of  disease  in  man,  and  in  part  from  experimental  removal 
in  animals. 

The  office  of  the  cerebral  hemispheres  as  a  whole  is  to 
receive  nerve  impulses  from  sensory  fibres,  which  awaken  what 
is  termed  perception,  and  give  rise  to  varied  sensations.  As  a 
result  of  these  sensations,  efferent  voluntary  impulses  may  be 
sent  out  from  the  cells  of  the  cerebrum,  and  give  rise  to  move- 
ments ;  or  contrariwise  nerve  impulses  may  be  sent  out  to  stop 


240  Elementary  Physiology, 

or  inhibit  movements  which  would  naturally  take  place  in  a 
purely   reflex   manner  but   for   the    restraining    and    guiding 
influence   of  the  cerebral  centres.     For  this   restraining  and 
guiding  activity  of  the  nerve  cells  of  the  cerebrum,  many  terms 
are  used  in  popular  language,  such  as  the  intelligence,  will,  and 
judgment.     When  the  cerebral  hemispheres  are  removed  com- 
pletely, the  animal  loses  all  this  intelligence  and  guiding  power. 
It  is,  like  a  ship  without  a  rudder,  completely  at  the  mercy  of 
all  those  influences  which  play  upon  it  from  the  outer  world. 
Each  stimulus  that  reaches  it  gives  rise  to  a  certain  response, 
just  as  if  the  animal  were  a  piece  of  complicated  but  unin- 
telligent  mechanism.      In  the   case    of  most    warm-blooded 
animals,  death  soon  follows  complete  removal  of  the  cerebral 
hemispheres ;  ^  but,  in  the  frog,   removal  is  not  followed  by 
death   for   a   long  time,   if  only  the   animal   be   kept  moist. 
Such  an  animal,  to  casual  observation,  appears  to  differ  little 
from  a  normal  frog ;  it  sits  in  a  normal  manner,  and  moves 
when  irritated  in  any  way.     If  placed  on  its  back,  it  at  once 
turns  over  into  the  natural  position.     If  thrown  into  a  vessel 
containing  water,  it  swims  in  a  regular  fashion  till  it  reaches 
the  side  of  the  vessel,  and  then,  if  possible,  will  crawl  up  this 
and  perch  passively  on  the  top  until  it  is  again  disturbed.     If 
placed  on  a  board  which  is  slowly  inclined,  it  does  not  slide  off 
as  the  inclination  is  increased,  but  balances  itself  and  crawls  up 
the  board,  to  perch  in  a  more  comfortable  position  on  the  top 
as  the  inclination  is  increased.     If  placed  in  front  of  an  opaque 
obstacle  and  stimulated  to  jump,  it  springs  to  one  side  so  as  to 
avoid   the    obstacle.     If  stroked   along   the  flanks,  it  croaks 
regularly.     In  fact,  the  animal  is  capable  of  carrying  out  the 
most  complicated  acts  in  a  perfectly  natural  manner.     But  it  is 
7nerely  an  automaton^  all  these  things  are  performed  mechanically, 
and  with  a  stimulus  definite  in  amount  and  kind,  it  is  perfectly 

*  That  the  case  is  much  the  same  in  mammals  is  shown  by  the  fact  that 
serious  injury  to  large  areas  of  the  cerebral  cortex  by  operation  leads  to  an 
idiotic  condition  of  the  animal,  as  is  also  the  case  when  the  blood-supply  to 
the  hemispheres,  and  hence  their  functional  activity  is  interfered  with. 
Again,  in  the  case  of  man,  want  of  development  or  insufficient  development 
of  the  cerebral  hemispheres  causes  idiotcy,  and  among  different  races  of 
men,  the  grade  of  intelligence  varies  directly  with  the  development  of  the 
cerebral  hemispheres. 


The  Nervous  System.  241 

settled  what  the  frog  will  do ;  there  is  but  one  answer  to  each 
stimulus.  It  never  does  anything  vohmtarily.  It  never  moves 
of  its  own  accord,  but  sits  continually  in  the  same  spot  and 
attitude  unless  stimulated  from  without.  It  takes  no  food, 
seems  to  have  no  sensations,  and  if  undisturbed  will  die  and 
dry  up  in  the  same  exact  position.  All  its  responses  to  stimuli 
are  definitely  measured  out  so  that  a  certain  strength  of  stimulus 
gives  a  certain  effect,  there  is  no  inhibition  of  movement  or 
origination  of  movement  by  a  reasoning  centre.  Swimming  is 
caused  by  the  afferent  nervous  impulses  started  by  contact  with 
the  water,  and  will  go  on  automatically  till  the  animal  reaches 
the  edge  or  until  it  sinks  exhausted  to  the  bottom ;  the  move- 
ments are  uncontrolled.  Again,  when  the  flanks  are  5troked^ 
the  animal  cannot  decide  to  croak  or  not  croak  like  a  complete 
frog,  but  must  croak  automatically  and  in  measured  degree 
each  time  it  is  stimulated. 

Similar  experiments  have  been  performed  on  the  pigeon 
with  like  results ;  the  animal,  when  undisturbed,  sits  as  if  asleep  ; 
but,  when  disturbed,  can  walk,  fly,  perch,  and  balance  itself  in 
a  normal  manner.  It  never  takes  food  voluntarily,  but  swallows 
mechanically  when  food  is  placed  at  the  back  of  the  throat, 
and  may  be  kept  alive  in  this  fashion  for  a  long  time. 

It  has  already  been  mentioned  that  there  is  localization  of 
function  in  different  parts  of  the  cortex  of  the  cerebral  hemi- 
spheres, and  that  the  cortex  around  the  fissure  of  Rolando 
governs  the  voluntary  movements  of  the  muscles  of  the  trunk 
and  limbs.  It  has  further  been  discovered  that  certain  other 
portions  of  the  cortex  are  concerned  in  special  sensations  and 
special  movements.  These  areas  are  shown  in  the  accompany- 
ing diagram  (Fig.  112),  and  have  been  localized  by  obser\ing 
the  effects  of  disease  or  injury  upon  these  parts,  and  by  experi- 
mental stimulation  or  removal  of  these  parts  of  the  cortex  in 
animals.  The  part  marked  "  lips  and  tongue  "  is  concerned  in 
controlling  the  movements  of  the  muscles  of  the  lips  and  tongue, 
and  of  regulating  speech.  Injury  to  this  part  causes  inco-ordi- 
nation  of  speech  and  inability  either  to  recollect^  or  it  may  be  to 
say  certain  words,  a  condition  which  is  termed  aphasia.  This 
part  of  the  cerebral  cortex  is  hence  termed  occasionally  the 

R 


242  Elementary  Physiology. 

"  speech  centre."  It  is  usually  the  left  side  of  the  brain  which 
normally  exercises  this  control ;  but  in  the  event  of  permanent 
injury  to  this  portion  of  the  cortex  on  the  left  side,  the  right 
side  after  a  time  takes  up  the  work,  and  the  aphasia  disappears. 
Certain  important  nerve  centres  lie  in  the  spinal  hUb  or  medulla 
oblojigata,  and  regulate  the  discharge  of  nerve  impulses  which 
are   essential  to  the  life  of  the  animal.      For   example,   the 


Fig.  112. — Diagram  of  the  external  surface  of  th2  brain  seen  from  the  left  side  ;  to 
indicate  the  position  of  the  chief  centres  of  localization. 

respiratory  centre  is  situated  here  which  regulates  respiration. 
If  the  medulla  be  destroyed  in  the  region  of  this  centre,  re- 
spiration ceases,  and  the  animal  promptly  dies  of  suffocation 
or  asphyxia.  The  afferent  impulses  arrive  chiefly  by  the 
vagus,  and  the  efferent  impulses  are  sent  out  along  the  phrenic 
nerves  which  supply  the  diaphragm,  and  the  intercostal  nerves 
which  supply  the  intercostal  muscles.  The  centre  is  acted 
upon  in  two  chief  ways  :  first,  chemically ^  by  the  nature  of  the 


The  New  Otis  System.  243 

blood  circulating  through  it,  for  when  this  is  poor  in  oxygen 
the  cells  of  the  respiratory  centre  are  stimulated  to  greater 
activity,  and  the  rhythm  of  respiration  is  quickened  ;  and 
secondly,  the  centre  is  stimulated  by  the  number  and  strength 
of  the  afferent  nerve  impulses  reaching  it.  When  the  respiratory 
efforts  are  artificially  increased  the  nerve  impulses  reaching  the 
centre  become  feebler,  and  the  centre  acts  less  energetically ; 
on  the  other  hand,  diminished  respiration,  or  suspended  re- 
spiration, for  a  short  time  strengthens  the  afferent  impulses 
sent  to  the  centre,  and  the  desire  to  breathe,  and  to  breathe 
strongly,  "becomes  imperative.  The  respiratory  centre  is  also 
affected,  and  the  rhythm  changed  by  nervous  impulses  reaching 
the  centre  along  various  other  channels.  Thus,  violent  emotions 
alter  the  respiration,  and  again  a  plunge  into  cold  water  will 
awaken  sensory  impulses  from  the  stimulation  of  the  skin, 
which  affect  the  respiratory  centre  and  change  the  rhythm. 
Quiet  normal  respiration  is,  however,  brought  about  by  an 
automatically  repeated  reflex,  which  is  regularly  discharged 
from  the  respiratory  centre,  and  the  variations  in  rhythm  are 
due  to  extra  afferent  impulses  arising  from  temporary  causes. 

Near  the  respiratory  centre  lies  also  the  important  vaso-motor 
centre  which  controls  the  tonicity  of  the  small  blood-vessels 
and  so  the  distribution  of  the  blood-stream.  Here,  also,  are 
situated  the  cardiac  centres,  which  control  the  rate  of  the  heart- 
beat. Other  centres  are  situated  in  the  medulla,  which  control 
the  reflexes  for  the  winking  or  closing  of  the  eyelids,  which  pre- 
serves the  surface  of  the  eyeball  clean  ;  which  control  reflexly 
the  diameter  of  the  pupil  of  the  eye,  and  so  regulate  the  amount 
of  light  entering ;  which  control  the  reflex  acts  of  swallowing, 
coughing,  sneezing,  secretion  of  saliva,  vomiting,  etc.  It  must 
not  be  understood,  however,  that  groups  of  nerve  cells  corre- 
sponding to  these  centres  have  been  demonstrated,  it  is  merely 
known  that  the  medulla  is  the  part  of  the  central  nervous 
system  which  regulates  these  reflex  acts. 

To  sum  up,  then,  the  nervous  system  is  an  exceedingly 
complex  network  of  nerve  cells,  nerve  fibres,  and  nerve  endings. 
The  nerve  cells  are  situated  centrally,  and  the  nerve  endings 
peripherally,  and  the  two  are  connected  by  nerve  fibres  passing 


244  Elementary  Physiology. 

between  centre  and  periphery.  Further,  the  nerve  cells  in  the 
different  parts  of  the  central  system  are  connected  together  in  a 
most  complex  fashion,  by  nerve  fibres  running  from  one  part  to 
another,  and  setting  the  whole  in  close  communication. 

In  the  spinal  cord  are  nerve  centres,  which  are  capable  of 
controlling  complicated  muscular  movements,  and  these  are 
under  the  control  of  the  nerve  cells  in  the  cerebral  cortex,  by 
which  they  may  either  be  set  in  motion  or  inhibited,  i.e.  put 
out  of  action.  In  the  medulla  (that  part  joining  brain  and 
cord)  lie  other  centres  which  guide  the  rhythm  of  heart-beat 
and  respiration  as  well  as  centres  for  carrying  out  other  im- 
portant work.  In  the  cerebellar  cortex  lie  intermediate  cell 
stations  which  govern  the  co-ordination  of  voluntary  move- 
ments, and  in  the  cerebral  cortex  lie  the  master  cells,  which 
are  so  affected  by  the  sensory  impulses  coming  to  them  as  to 
give  rise,  in  some  way  unknown  to  us,  to  consciousness,  will, 
judgment,  intelligence,  and  memory,  and  have  the  power  of 
sending  out  in  reply  different  impulses  which  originate  the  acts 
of  the  animal. 


CHAPTER   XII. 

THE  SENSES. 

Our  sensations  are  awakened  by  the  stimulation  of  nerve 
centres  in  the  brain  by  nerve  impulses  which  start  by  the 
excitation  of  sensory  nerve  endings  in  the  periphery  and 
travel  to  the  brain  along  sensory  or  afferent  nerve  fibres. 

It  is  usual  to  divide  sensations  into  two  classes,  although 
there  is  no  very  essential  difference  in  kind  corresponding  to 
the  classification.  These  two  classes  are  common  sensations 
and  special  sensations. 

The  first  class  includes  those  general  sensations  which 
cannot  be  localized  accurately,  such  as  fatigue,  hunger,  and 
thirst,  as  well  as  the  sensations  which  give  rise  to  coughing, 
vomiting,  tingling,  itching,  and  such  like.  All  these  sensations 
are  referred  to  changes  going  on  within  the  body. 

The  muscular  sense  is  also  usually  classed  as  a  common 
sensation.  This  sense  gives  an  impression  of  the  state  of 
contraction  of  the  skeletal  muscles,  and  so  enables  the  nervous 
system  to  regulate  the  degree  of  contraction  which  is  necessary 
in  the  various  groups  of  opposed  muscles  in  carrying  out 
compUcated  movements,  such  as  walking,  grasping,  writing, 
etc.  When  the  muscles  contract  against  resistance  the  muscular 
sense  also  gives  an  idea  of  the  amount  of  effort  required,  and 
it  is  this  sense  which  we  employ  when  we  weigh  bodies  in  the 
hand  by  movements  of  the  forearm.^  It  is  probable  that  the 
sensory  nerve  endings  for  the  muscular  sense  lie  in  the  muscles 

*  The  muscular  sense  is  much  more  delicate  for  this  purpose  than  is  the 
sense  of  pressure  alone.  Thus,  while  a  difference  of  weight  of  one  in 
thirty  is  easily  appreciated  by  most  people  when  movement  of  the  forearm 
is  allowed,  it  is  scarcely  possible  to  detect  a  difierence  of  weight  of  less 
than  one  in  eight  when  pressure  on  the  palm  is  alone  employed. 


246  Elementary  Physiology. 

themselves,  although  it  has  been  held  by  some  that  sensory 
nerve  endings  in  the  articular  surfaces  of  the  joints  have  a 
ofreat  deal  to  do  with  the  sense  of  muscular  effort,  and  with 
our  appreciation  of  the  weights  of  bodies. 

Certain  sensory  nerves  are,  however,  connected  with  specific 
kinds  of  sensations,  which  are  produced  by  influences  outside 
the  body,  and  these  sensations  are  termed  special  sensations. 
Usually  five  special  senses  are  recognized,  which  are  popularly 
known  as  the  ^'' five  senses;''  these  are  totich,  taste^  smelly  hearings 
and  sight. 

The  sense  of  touch  lies  on  the  borderland  between  common 
and  special  sensations.  It  differs  from  the  other  four  of  the 
five  senses  in  that  it  is  not  conveyed  to  the  brain  by  any 
special  nerve,  but  may  be  awakened  by  stimulation  of  any  of 
the  nerve  endings  of  touch  in  connection  with  the  cutaneous 
nerves.  But  it  resembles  the  other  four  special  sensations  in 
that  the  cause  producing  it  is  referred,  to  somewhere  and  some- 
thing outside  the  body,  to  an  external  cause.  When  we  touch 
an  object  belonging  to  our  surroundings,  the  sensation  pro- 
duced is  known  to  be  caused  by  something  outside  the  body; 
just  as  when  we  see  an  object,  we  are  aware  that  the  visual 
sensation  is  awakened  by  something  in  the  external  world. 
But  when  our  tooth  aches,  or  when  we  are  wearied,  we  feel  that 
it  is  something  connected  with  our  body  which  is  giving  rise  to 
the  sensation. 

We  have  to  deal  here  more  particularly  with  the  special 
sensations  and  the  apparatus  by  which  they  are  evoked;  we 
have  to  describe  in  outline  the  nature  of  the  peripheral  organs 
by  means  of  which  the  various  forms  of  energy  reaching  the 
body  from  the  outer  world  are  enabled  to  awaken  sensory 
nerve  impulses,  which,  travelling  along  definite  paths  to  definite 
parts  of  the  cerebrum,  awaken  in  our  consciousness  each  its 
own  specific  sensation. 

There  are  one  or  two  common  properties  of  the  special 
sensations  which  it  may  be  well  to  state  before  passing  to  the 
description  of  each  of  the  peripheral  organs. 

The  nerve  fibres  which  pass  from  a  peripheral  special  sense 
organ  are  only  capable  of  conveymg  to  the  consciousness  one 


TJie  Senses.  247 

specific  kind  of  sensation.  This  is  known  as  the  law  of 
specific  sensation.  Thus  the  optic  nerves  when  stimulated  in 
any  manner  only  give  rise  in  the  consciousness  to  visual 
sensations,  although  the  stimulation  may  be  affected  in  various 
ways,  such  as  ordinarily  by  the  stimulation  of  the  proper  nerve 
endings  of  the  optic  nerves  by  light ;  pressure  on  the  eyeball 
when  the  eyelids  are  closed ;  ^  application  of  the  electric  current, 
or  severing  the  optic  nerve.  The  same  is  true  of  each  of  the 
other  specific  sensations.  For  example,  irritation  of  the  mucous 
membrane  of  the  nasal  passages  where  the  olfactory  nerve 
endings  lie,  as  by  a  catarrh  or  cold,  may  produce  subjective 
sensations  ^  of  various  smells. 

Whatever  part  of  a  sensory  nerve  fibre  be  irritated,  the 
sensation  is  always  referred  by  the  consciousness  to  the  nerve 
ending  in  the  periphery.  Thus,  after  an  amputation,  when  the 
cut  ends  of  the  nerve  by  reason  of  the  irritation  send  impulses 
to  the  brain,  these  are  felt  as  coming  from  the  cut-off  part,  and 
the  patient  believes  he  feels  pain  in  his  fingers  or  toes  as  if 
these  still  formed  part  of  his  body.  Again,  if  the  elbow  be 
immersed  in  a  freezing  mixture,  the  chill  irritates  the  nerve 
trunks  of  the  arm  at  the  elbow ;  but  the  sensation  experienced 
is  not  felt  as  coming  from  the  elbow,  a  sensation  of  pain  instead 
is  felt  which  is  referred  to  the  finger-tips.  The  same  kind  of 
thing  is  felt  when  the  ulnar  nerve,  w^hich  is  almost  subcutaneous 
at  the  elbow,  is  irritated  by  pressure  or  by  a  chance  blow; 
the  pricking  sensation  of  "  pins  and  needles  "  is  then  felt,  not 
at  the  seat  of  irritation,  but  all  the  way  down  the  forearm,  where- 
ever  the  ulnar  nerve  has  sensory  endings,  down  to  the  finger-tips. 
Another  example  of  the  peripheral  reference  of  sensation  by 
the  sensor i^ivi  is  the  peculiar  feeling  experienced  when  a  limb, 
as  it  is  popularly  termed,  "  goes  asleep!'  This  is  due  to  a  slight 
continued  pressure  at  any  point  on  the  nerve  trunk,  and  not  to 
anything  at  the  periphery,  although  the  tingling  sensation  is  felt 
all  over  the  distribution  to  the  limb  of  the  nerve  pressed  upon. 

^  The  coloured  images  so  produced  are  termed  phosphoies. 

-  A  subjective  sensation  is  one  produced  in  an  abnormal  fashion,  giving 
rise  to  an  impression  of  something  in  the  outer  world  which  has  no  exist- 
ence in  fact. 


248  Elementary  Physiology. 

It  has  therefore  to  be  borne  constantly  in  mind  that  the  seat  of 
injury  giving  rise  to  a  pain  or  other  sensation  may  be  either 
central^  or  in  the  7ierve  trunks  or  peripheral ;  and  still,  in  any  of 
these  cases,  the  nerve  centres  can  only  act  and  give  rise  to 
consciousness  by  sending  in  nerve  impulses  by  the  usual 
channels.  Hence  these  are  estimated  as  rising  in  the  accus- 
tomed fashion,  and  so  give  rise  in  the  consciousness,  both  to 
the  specific  sensation  which  is  normally  derived  from  impulses 
carried  by  that  particular  nerve  route,  and  to  the  impression 
that  the  sensation  has  been  awakened  in  a  normal  fashion  by 
stimulation  at  the  peripheral  nerve  endings. 

The  strength  of  sensations  is  not  proportional  directly  to  the 
strength  of  the  physical  stimulus,  but  varies  as  the  logarithms  ^ 
of  the  physical  strengths  ;  this  is  known  as  the  Weber-Feehner  law. 
It  would  never  do,  for  example,  if  a  light  equivalent  to  1000 
candles  in  the  physical  intensity  of  its  illumination  produced  one 
thousandfold  the  effect  on  the  eye.  In  spite  of  the  arrangements 
in  the  eye,  which  will  subsequently  be  described,  for  shutting  out 
excessive  light,  if  such  a  condition  of  things  existed,  the  bright 
light  would  only  produce  pain  by  excessive  stimulus,  and  an 
illumination  much  short  of  the  electric  light,  and  far  short  of 
that  of  a  bright  sunny  day,  would  absolutely  blind  us ;  either 
that  or  we  could  not  see  by  the  rushlight  or  starlight.  But  the 
relationship  stated  above  of  stimulation  to  sensation,  tends  to 
equalize  sensations  so  that  we  can  bear  and  appreciate  both 
strong  and  weak  stimuli.  To  a  certain  extent  this  toning  down 
has  to  be  paid  for  in  inability  to  distinguish  small  variations  in 
intensity,  but  the  acuteness  in  this  direction  is  sufficient  for  the 
ordinary  wants  of  our  life. 

Although  subjective   sensations,  as  stated  above,  may  be 
felt  when  the  sensorium  is  divided  from  the  peripheral  nerve 

^  For  those  unacquainted  with  the  nature  of  logarithms,  a  simpler 
statement  of  the  law  is  that  the  change  in  magnitude  of  the  sensation  is 
proportional  to  percentage  increase  of  the  physical  stimulus.  Thus,  a 
minute  trace  of  difference  in  intensity  of  illumination  can  be  made  out  by 
most  persons  between  two  lights  of  100  candles  and  loi  candles  respec- 
tively ;  now,  to  see  such  a  difference  with  an  illumination  equal  to  1000 
candles,  alight  not  of  100 1  candles,  but  of  10 10  candles,  must  be  employed 
in  comparison. 


The  Senses.  249 

ending,  or  when  a  different  part  of  the  system  than  the  nerve 
ending  is  irritated,  still  no  objective  ^  sensation  can  arise 
except  by  action  in  a  normal  manner  of  the  proper  kind  of 
stimulus  on  the  nerve  endings,  when  these  are  in  connection 
physiologically  with  the  sensorium,  and  when  all  the  entire 
physiological  system  (of  nerve  endings,  connecting  machinery 
of  nerve  fibres,  and,  it  may  be,  intermediate  nerve  cells,  and 
central  nerve  cells)  is  in  working  order. 

For  example,  we  can  only  see  an  object,  which  has  a  real 
existence  apart  from  our  fancy,  and  the  existence  of  which  can 
be  corroborated  by  our  other  senses,  when  it  is  illuminated  and 
casts  an  image  on  the  retina  at  the  back  of  the  eyeball,  thus 
awakening,  in  some  unknown  manner,  nerve  impulses  in  the 
fibres  of  the  optic  nerve,  which  travel  finally  to  nerve  cells  in 
the  occipital  cortex  of  the  cerebrum  and  set  these  in  activity. 
Injury  to  any  part  of  the  physiological  chain  between  retina 
and  cerebral  nerve  cell  may  cause  blindness.  For  example, 
the  retina  may  be  insensitive  from  some  cause,  and  then  no 
object  can  be  seen,  although  the  rest  of  the  visual  machinery 
may  be  quite  perfect.  Under  such  conditions,  subjective  sen- 
sations may  indeed  be  produced  by  stimulation  of  the  trunk  of 
the  optic  nerve,  but  it  cannot  be  stimulated  by  light,  and  no 
object  in  the  external  world  can  be  discerned.  Again,  the  eye 
may  be  uninjured,  but  the  optic  nerve  or  optic  tract  either  cut 
across,  or  for  some  reason  inoperative  as  a  conductor,  and  the 
result  is  blindness.  Finally,  the  cerebral  area  for  vision  (see 
Fig.  112)  may  be  removed  or  diseased,  and  again  the  effect  is 
the  same. 

Cutaneous  Sensations. 

The  skin  over  the  entire  surface  of  the  body  is  supplied 
with  special  sensory  nerve  endings.  These  nerve  endings  have 
many  different  forms  in  different  regions  of  the  skin,  and  also  in 
different  animals.  One  of  the  commonest  forms  is  the  tactile 
corpuscle  (see  Fig.  no,  p.  236) ;  these  occur  in  immense  numbers 
lying  in  rows  immediately  beneath  the  epidermis  in  the  papillae 

^  That  is,  a  sensation  caused  by  something  with  a  real  existence  in  the 
external  world. 


250  Elementary  Physiology. 

of  the  skin  of  the  hand  and  foot.  The  tactile  corpuscles  are  so 
called  because  they  are  supposed  to  be  specially  connected 
with  the  sense  of  touch,  and  certainly  they  are  found  in  greatest 
abundance  where  the  sense  of  touch  is  most  acute.  Still,  there 
are  other  cutaneous  sensations  besides  tactile  sensation,  and  it 
is  by  no  means  clearly  proved  that  the  tactile  corpuscles  are 
not  connected  with  the  appreciation  of  these  other  sensations 
as  much  as  with  that  of  touch.  Stimulation  of  the  skin  of  any 
area  by  appropriate  means  may  give  rise  to  sensations,  either 
of  heat,  of  cold,  of  touch,  or  of  pain,  and  there  is  a  certain 
amount  of  evidence  that  these  different  sensations  are  produced 
by  stimulation  of  different  and  specific  nerve  endings.  For 
example,  if  a  pointed  rod  of  iron,  which  is  either  considerably 
hotter  or  considerably  colder  than  the  temperature  of  the  body, 
be  made  to  touch  successively  various  points  on  the  skin,  it  will 
be  found  that  certain  points  called  "  heat  points "  exist,  at 
which  the  hot  point  is  much  more  acutely  appreciated  than  at 
others;  similarly  there  are  other  points  at  which  the  chilled 
point  gives  rise  to  a  much  more  acute  sensation  of  cold,  and 
these  "  cold  spots  "  do  not  coincide  with  the  "  heat  spots." 
Hence  the  more  acute  perception  does  not  depend  solely  on 
closeness  to  nerve  endings,  and  there  must  be  different  nerve 
endings,  some  sensitive  to  a  low  and  some  to  a  high  tempera- 
ture. The  perception  of  difference  in  temperature  by  the 
skin  is  often  spoken  of  as  the  temperature  sense.  This  by  no 
means  corresponds  in  its  delicacy  at  different  parts  with  that 
of  the  tactile  sense,  for  tactile  sensation  is  most  acute  at  the 
tip  of  the  tongue  or  the  tips  of  the  fingers ;  while  the  tempera- 
ture sense  is  most  acute  upon  the  skin  of  the  cheek,  where  the 
tactile  sensation  is  not  nearly  so  delicately  developed.  It  must 
hence  be  admitted  that  the  tactile  sense  and  the  temperature 
sense  are  distinct  from  each  other,  and  are  probably  furnished 
by  quite  different  sets  of  nerve  endings  and  nerve  fibres. 

The  delicacy  of  the  sense  of  touch  in  different  regions  of 
the  skin  is  estimated  by  the  smallest  distance  apart  at  which 
two  points  are  still  felt  as  distinct.  The  testing  is  carried  out 
by  a  pair  of  compasses  so  blunted  as  not  to  prick  the  skin  or 
give  rise  to  sensations  of  pain.     It  is  found  by  this  method  that 


The  Senses.  251 

the  two  points  can  still  be  appreciated  as  distinct  when  applied 
to  the  tip  of  the  tongue  at  a  distance  apart  of  about  i  milli- 
metre ;  at  a  less  distance  than  this  the  points  are  no  longer 
felt  as  two,  and  the  person  experimented  upon  is  unable  to  say 
whether  both  points  are  applied  to  the  skin  or  only  one  by 
the  person  carrying  out  the  test.  The  tips  of  the  fingers  are 
next  in  order  of  delicacy  j  here  the  points  can  still  be  felt  as 
discrete  when  only  2  millimetres  apart.  Other  parts  are  less 
sensitive ;  for  example — lip,  9  millimetres ;  front  of  forearm, 
15  millimetres;  forehead,  23  millimetres;  back  of  hand,  30 
millimetres;  neck,  back,  arm,  and  thigh,  50-70  millimetres. 

It  is  often  erroneously  stated  that  the  sensation  of  touch  is  due  to 
pressure  on  the  sensory  nerve  endings  in  the  skin  ;  it  would  be  much  more 
correct  to  say  that  tactile  sensation  is  due  to  variation  in  pressure  upon 
these  nerve  endings.  Constant  localized  pressure  on  the  skin  if  excessive 
may  give  rise  to  a  sensation  of  pain,  but  when  not  excessive  does  not  give 
rise  to  tactile  sensation  if  constantly  applied.  It  is  the  sudden  application 
of  pressure  or  removal  of  pressure  which  affects  the  tactile  nerve  endings, 
and  gives  rise  to  the  consciousness  of  something  touching  the  part.  This 
may  be  shown  by  laying  a  light  object  over  the  finger,  which  is  kept  as  still 
as  possible  by  resting  it  upon  a  table.  After  a  short  time  the  light 
object  is  scarcely  felt  to  touch  the  finger,  being  only  appreciated  by  the 
excessively  slight  involuntary  movements  and  by  the  pulsations  of  the  blood 
in  the  finger,  which  cause  slight  variations  in  pressure.  If  now  the  table 
be  tapped,  or  the  finger  slightly  moved,  the  object  is  felt  much  more  dis- 
tinctly. The  same  thing  is  experienced  when  the  finger  is  dipped  into  a 
vessel  of  mercury.  Here  a  very  distinct  sensation  of  touch  is  experienced 
where  the  surface  of  mercury  touches  in  a  ring  around  the  finger,  but  little 
tactile  sensation  is  awakened  at  the  tip  of  the  finger  which  is  deepest  in  the 
mercury  and  has  the  most  delicate  development  of  the  tactile  sensation. 
This  shows  that  it  is  not  the  pressure  which  arouses  tactile  sensation,  for 
this  is  greatest  at  the  finger-tip,  but  the  rapid  variation  or  oscillation  in 
pressure  which  is  greatest  at  the  surface  of  the  mercury  where  slight  move- 
ments of  the  finger  are  continually  alternately  increasing  and  decreasing 
the  pressure. 

The  mtaneous  sensation  of  pain  is  probably  specially 
localized  in  certain  nerve  endings  and  fibres  like  the  sensa- 
tions of  heat  and  cold ;  for  by  a  somewhat  similar  method  to 
that  which  was  employed  for  demonstrating  "  heat  spots  "  and 
"  cold  spots "  (namely,  gently  pricking  the  skin  with  a  sharp 
point),  it  may  be  shown  that  certain  localized  points  are  much 


252  Elementary  Physiology. 

more  sensitive  than  others.  At  the  same  time,  sensations  of 
pain  can  be  elicited  from  any  point  on  the  skin  if  the  prick  be 
severe  enough,  and  also  sensations  of  pain  can  be  evoked  by 
stimulation  of  nerves  which  do  not  supply  cutaneous  areas,  and 
generally  it  seems  to  be  the  case  that  excessive  stimulation  of 
sensory  nerve  fibres  anywhere  is  capable  of  awakening  unpleasant 
sensation  which  is  indefinitely  and  vaguely  spoken  of  as  pain. 

There  is  no  doubt  that  under  the  general  title  of  pain 
many  sensations  are  included  together  which  are  really  distinct 
in  character,  and  nearly  any  sensation  which  becomes  suffi- 
ciently unpleasant  is  termed  pain.  For  example,  the  pain  of 
a  headache  is  different  in  character  from  that  of  a  toothache, 
and  both  are  different  from  that  of  a  burn.  So  that  pain  is 
not  a  specific  sensation,  but  rather  a  term  used  to  include 
certain  somewhat  closely  allied  forms  of  unpleasant  irritation 
of  the  nervous  system  which  usually  arise  from  excessive  stimu- 
lation of  sensory  nerves. 

Taste  and  Smell. 
The  end  organs  of  taste  are  situated  chiefly  on  the  tongue 
and  soft  palate.  The  terminal  ramifications  of  the  gustatory 
nerves  pass  to  small  ovoid  clumps  of  cells  which  are  known  as 
'*  taste  buds."  The  taste  buds  (see  Fig.  113)  are  best  seen  in 
the  depressions  of  the  cirmmvallate  papillce,  which  are  elevations 
arranged  in  a  somewhat  V-shaped  manner  at  the  base  of  the 
tongue,  but  they  are  also  to  be  found  all  over  the  tongue,  and 
upon  the  under  surface  of  the  soft  palate,  lying  imbedded  in  the 
stratified  epithelium.  There  are  two  kinds  of  cells  found  in  each 
taste  bud,  viz.  the  gustafo7y  cells,  which  are  slender  and  fusiform, 
with  a  prominent  nucleus  and  a  long  process  at  each  end  ;  and 
the  siLstenfaadar  cells,  which  are  elongated,  flattened  cells, 
pointed  at  their  ends.  The  sustentacular  cells  lie  between, 
and  appear  to  support,  the  more  delicate  gustatory  cells,  and 
also  form  a  cortical  envelope  round  the  outer  part  of  the  taste 
bulb.  It  is  probable  that  the  gustatory  cells  are  those  which 
are  affected  by  the  dissolved  ^  sapid  substance,  and  that  this 

'  Only  substances  in  solution  affect  the  taste  organs  ;  thus,  for  example, 
quinine  in  powder  is  insoluble  on  the  tongue  and  has  scarcely  any  taste, 
although  quinine  in  solution  is  intensely  bitter. 


TJie  Senses. 


253 


alteration  starts  nerve  impulses  in  the  terminations  of  the 
gustatory  nerves  which  ramify  round  the  gustatory  cells.  The 
nerves  of  taste  are  the  glosso-pharyngeal,  which  supplies  the 
posterior  part  of  the  tongue  and  the  soft  palate,  and  the  Ungual 
branch  of  the  fifth  cranial  nerve  and  the  chorda  tympani, 
which  supply  the  anterior  part  of  the  tongue/    In  some  persons 


'f^y^'7.'  \W^^S'^'^^ 


Fig.  113. — Section  through  the  middle  of  a  taste  bud.     CRanvier.) 

/,  gustatory  pore  ;  5,  gustatory  cell  ;   r,  sustentacular  cell ;   m,  lymph  cell,  containing 

fatty  granules  ;  e,  superficial  ceils  of  the  stratified  epithelium  ;  «,  nerve  fibres. 

no  taste  sensations  whatever  can  be  appreciated  on  the  tip  of 
the  tongue ;  while  other  individuals  can  only  taste  sweet  sub- 
stances in  this  region.  The  distribution  of  the  different  kinds 
of  taste  sensation  is  not  uniform,  and  varies  in  different  indi- 
viduals. Most  usually,  sweet  substances  are  tasted  at  the  tip, 
and  bitter  substances  at  the  back  of  the  tongue ;  but  some 
persons  have  both  sweet  and  bitter  tastes  at  the  tip  as  well  as 
at  the  base. 

It  is  usual  to  state  that  there  are  four  primitive  types  of  taste  sensation, 
viz.  sweet,  bitter,  salt,  and  sour,  but  it  is  doubtful  whether  all  possible 


^  It  is  probable  that  all  gustatory  fibres  arise  from  the  root  of  the  fifth 
nerve,  and  join  the  glosso-pharyngeal  and  chorda  tympani  afterwards. 


254 


Elementajy  Physiology. 


tastes  can  be  referred  to  one  of  these  four  classes.  Still,  it  is  certain  that 
the  many  different  fiavotcrs  which  we  experience  in  the  different  foods  we 
eat  are  to  a  great  extent  due  to  a  combination  of  olfactory  sensations  with 
gustatory  sensations.  It  is  a  matter  of  common  experience  that  when  the 
olfactory  mucous  membrane  is  temporarily  thrown  out  of  working  order  by 
a  severe  cold  in  the  head  or  nasal  catarrh  that  we  not  only  lose  the  sense 

of  smell,  but  that  the  sense  of  taste  also  suffers, 
and  we  can  no  longer  properly  taste  our  food. 
This  is  due  to  the  absence  of  the  olfactory 
stimuli  which  previously  formed  a  complex 
with  the  simpler  sensations  of  taste,  and  gave 
rise  to  the  flavour ;  in  other  words,  we  are 
unable  to  enjoy  the  flavour  of  our  food  in  such 
a  condition,  because  we  cannot  smell  it  as 
well  as  taste  it. 

Sensations  of  smell  arise  from  the 
stimulation  of  certain  cells  lying  in 
the  mucous  membrane  lining  the  upper 
portion  of  the  nasal  passages.  There 
are  two  distinct  kinds  of  epithelium 
found  in  the  mucous  membrane  of 
the  nasal  passages ;  that  lining  the 
lower  part  {ScJmeiderian  membrane)  is 
ciliated  like  the  epithelium  of  the 
trachea,  while  that  of  the  upper  part 
{olfactory  membrane)  is  chiefly  colum- 
nar and  devoid  of  cilia.  The  air 
passing  in  and  out  to  the  lungs  through 
resrion       (M.    Schuitze.)   ^^    nasal   passasfcs   passes    over   the 

(Highly  magnified.)  ^  ... 

part  lined  with  ciliated  epithelium, 
and  this  portion  is  not  sensitive  to 
smell.  It  is  only  when  the  molecules 
of  the  odorous  substance  are  carried 
upwards  by  diffusion  or  air  currents  to 
the  olfactory  mucous  membrane  that 
the  sense  of  smell  is  awakened.  This  takes  place  much  more 
rapidly  when  inspiration  is  forced,  and  hence  it  is  that  we 
sniff  or  take  in  the  air  in  little  rapid  inspiratory  gusts  when 
we  attempt  to  detect  a  smell.  The  ultimate  end  organs  of 
smell  are   probably  spindle-shaped  or  bi-polar  ceUs  which  lie 


Fig.    114 


Cells    and    terminal 
nerve  fibres  of  the  olfactory'- 


from  the  frog ;  2,  from  man  ; 
a,  epithelial  cell,  extending 
deeply  into  a  ramified  pro- 
cess ;  b,  olfactory  cells ;  c, 
their  peripheral  rods ;  e, 
their  extremities,  seen  in  i 
to  be  prolonged  into  fine 
hairs  ;  d,  their  central  fila- 
ments. 


The  Senses.  255 

interposed  between  the  columnar  cells  of  the  olfactory  mucous 
membrane  (see  Fig.  114).  These  cells  are  termed  olfadoiy 
cells,  while  the  columnar  cells  which  appear  to  serve  to  support 
them  are  called  siistefitamlar  cells.  One  process  of  the  olfac- 
tory cell  projects  towards  the  free  surface,  and  in  some  classes  of 
animals  ends  in  a  number  of  minute  hairs.  The  other  process 
is  long  and  delicate,  somewhat  resembling  a  non-medullated 
nerve  fibre  and  becomes  lost  to  view  in  a  section  of  the 
membrane  among  the  plexus  of  olfactory  nerve  fibres,  lying 
immediately  beneath  the  epithelium.  The  olfactory  nerve  fibres 
are  non-medullated,  they  unite  to  form  from  twenty  to  thirty 
small  nerves  which  pierce  the  cribriform  plate  of  the  ethmoid 
bone,  and  so  enter  the  cranium  and  pass  to  the  olfactory 
lobes. 

Hearing. 

The  organ  of  hearing  is  usually  described  as  consisting 
of  three  parts,  viz.  the  external  ear,  the  middle  ear  or  tym- 
panum, and  the  internal  ear  or  labyrinth.  The  external 
and  middle  ear  are  accessory  parts  necessary  for  conveying 
the  vibrations  of  the  air,  or  sound  waves,  in  a  modified  form 
to  the  internal  ear  in  which  are  situated  the  terminations  of  the 
auditory  nerve  in  a  complicated  structure  known  as  the  07'gan 
of  Corti. 

The  external  eaj-  includes  the  pinna,  which  projects  from  the  head  and 
forms  what  is  termed  "the  ear"  in  popular  phraseology,  and  a  passage  (see 
Fig.  115)  which  leads  from  this  towards  the  middle  ear  called  the  external 
auditory  meatus.  The  pinna  is  composed  of  a  shell  of  elastic  cartilage 
covered  with  skin,  which  becomes  pendulous  at  the  lower  part  in  the  lobnle, 
and  contains  there  a  certain  amount  of  fat.  In  certain  of  the  lower  animals, 
the  pinnae  can  be  moved  about  by  attached  muscles,  and  besides  collecting 
the  sound  waves  like  an  ear-trumpet,  and  thus  making  the  hearing  more 
acute,  serve  to  give  information  of  the  direction  from  which  a  sound  is 
coming  by  the  direction  of  pointing  in  which  it  is  most  easily  heard.  But 
in  man  the  pinna  is  rudimentary,  and  is  more  ornamental  than  useful ;  for 
persons  who  have  been  deprived  of  their  ears  have  almost  normal  hearing  ; 
and  also  information  is  obtained  as  to  the  directions  of  sounds  not  by 
moving  the  ears,  although  vestiges  of  muscles  still  persist,  but  by  moving 
the  head . 

The  external  auditory  meatus  is  closed  at  its  internal  end  (see  Figs.  115, 


256 


Elementary  Physiology. 


117)  by  the  tympanic  membrane,  or  drum  of  the  ear,  which  is  placed 
obliquely  across  the  meatus  and  forms  part  of  the  external  wall  of  the  cavity 
of  the  tympanum. 

The  middle  chamber  of  the  ear,  called  the  tympanum,  is  a  small 
irregular  cavity  in  the  substance  of  the  temporal  bone.  It  contains  a  chain 
of  three  small  bones  called  the  auditory  ossicles  (see  Figs.  116,  117),  which 


Fig.  115. — Diagrammatic  view  from  before  of  the  parts  composing  the  organ  of  hearing  of 
the  left  side.     (After  Arnold.) 

The  temporal  bone  of  the  left  side,  with  the  accompanying  soft  parts,  has  been  detached 
from  the  head,  and  a  section  has  "been  carried  obliquely  through  it  so  as  to  remove 
the  front  of  the  meatus  externus,  half  the  tympanic  membrane,  and  the  upper  and 
anterior  wall  of  the  tympanum  and  Eustachian  tube.  The  meatus  internus  has  also 
been  opened,  and  the  bony  labyrinth  exposed  by  the  removal  of  the  surrounding 
parts  of  the  petrous  bone,  i,  the  pinna  and  lobule;  2  to  2',  meatus  externus; 
2',  membrana  tympani  ;  3,  cavity  of^  the  tympanum;  above  3,  the  chain  of  small 
bones  ;  3',  opening  into  the  mastoid  cells  ;  4,  Eustachian  tube  ;  5,  meatus  internus, 
containing  the  facial  (uppermost)  and  auditory  nerves  ;  6,  placed  on  the  vestibule  of 
the  labyrinth  above  the  fenestra  ovalis  ;  a,  apex  of  the  petrous  bone  ;  b,  internal 
carotid  artery  ;  c,  styloid  process  ;  d,  facial  nerve  issuing  from  the  stylo-mastoid 
foramen  ;  e,  mastoid  process  ;  /,  squamous  part  of  the  bone. 


serve  to  convey  with  diminished  amplitude  and  somewhat  modified  the 
vibrations  of  the  tympanic  membrane  to  the  internal  ear,  where  they 
produce  an  effect  on  the  endings  of  the  auditory  nerve.  The  auditory 
ossicles  are  kept  in  position  by  certain  slender  ligaments  attached  to  the 
bony  walls  of  the  tympanum,  and  their  tension  and  that  of  the  tympanic 
membrane  is  adjusted  by  certain  tiny  muscles.  The  most  external  of  the 
three  ossicles,  which  is  termed  the  malleus,  from  a  supposed  resemblance  to 


TJie  Se?tses. 


257 


a  hammer,  is  attached  by  its  longer  limb  to  the  tympanic  membrane  in  an 
eccentric  fashion  so  as  to  damp  the  vibrations  which  that  membrane  would 
tend  to  make  most  easily,  and  so  renders  it  equally  responsive  to  notes  of 
all  pitches.  The  intermediate  ossicle,  the  incus,  articulates  by  its  shorter 
limb  with  the  shorter  limb  of  the  malleus,  and  the  extremity  of  its  longer  limb 
is  attached  by  a  ligament  (in  which  a  tiny  ossicle  is  developed)  to  the  head 
of  the  stapes  (or  stirrup  ossicle).  The  base  of  the  stapes,  which  is  oval  and 
surrounded  by  cartilage,  is  attached  to  a  membrane  closing  an  opening  of 
an  oval  shape  situated  on  the  internal  wall  of  the  tympanum,  called  the 


Fig.  116. — View  of  the  left  membrana  tympani  and  auditory  ossicles  from  the  inner  side, 
and  somewhat  from  above  (E.  A.  S  ).     f 

7n,  malleus  ;  i,  incus  ;  st,  stapes  ;  ^y,  pyramid  from  which  the  tendon  of  the  stapedius 
muscle  is  seen  emerging  ;  1 1,  tendon  of  the  tensor  tympani  cut  short  near  its 
insertion  ;  I. a,  anterior  ligament  of  the  malleus:  the  processus  gracilis  is  concealed 
by  the  lower  fibres  of  this  ligament ;  l.s,  superior  ligament  of  the  malleus  ;  /.  i,  liga- 
ment of  the  incus  ;  ch,  chorda  tympani  nerve  passing  across  the  outer  wall  of  the 
tympanum. 


fenestra  ovalis.  The  fenestra  ovalis,  as  well  as  another  opening  on  the 
internal  wall  of  the  tympanum,  also  covered  by  membrane  and  called 
the  fenestra  rotundis,  communicates  with  the  internal  ear.  The  purpose 
of  these  two  communications  will  be  pointed  out  later. 

The  cavity  of  the  middle  ear  is  filled  with  air,  and  is  in  communication 
with  the  atmospheric  air  by  means  of  a  passage  or  tube  called  the 
Eustachian  tube,  which  opens  into  the  pharynx  (see  4,  Fig.  115).  The  purpose 
of  the  Eustachian  tube  is  to  keep  the  air  at  equal  pressure  on  both  sides  of 
the  tympanic  membrane,  so  that  this  may  be  free  to  vibrate,  and  not  be 
forced  in  or  out  by  difference  in  air-pressure  on  its  two  surfaces.     The  tube 

S 


258 


Elementary  Physiology, 


does  not  remain  open  always,  but  opens  to  adjust  the  pressure  when 
any  variation  takes  place  ;  it  opens  for  this  reason  during  the  act  of 
swallowing. 

The  internal  ear  or  labyrinth  lies  in  a  cavity  of  complicated   shape 
hollowed  out  in  the  substance   of  the  temporal  bone,   called  the  osseous 


i.aru.m 


au.m 


Fig.  117. — Profile  view  of  the  left  membrana  tympani  and  auditory  ossicles  from  before 
and  somewhat  from  above.     Magnified  four  times-     (E.  A.  S.) 

The  anterior  half  of  the  membrane  has  been  cut  away  by  an  oblique  slice,  m,  head  of 
the  malleus  ;  sp,  spur-like  projection  of  the  lower  border  of  its  articular  surface  ; 
pr.  br,  its  short  process  ;  pr.  gr,  root  of  processus  gracilis,  cut ;  s.l.fu,  suspensory 
ligament  of  the  malleus  ;  l.e.tn,  its  external  ligament  ;  t.t,  tendon  of  the  tensor 
tympani,  cut;  i,  incus,  its  long  process;  st,  stapes  in  fenestra  ovalis  ;  e.au.m, 
external  auditory  meatus  ;  p-R.  notch  of  Rivinus  ;  m.t,  membrana  tympani  ;  u,  its 
most  depressed  point  or  umbo  ;  d,  declivity  at  the  extremity  of  the  external  meatus  ; 
i.au.vt,  internal  auditory  meatus  ;  a  and  b^  its  upper  and  lower  divisions  for  the 
corresponding  parts  of  the  auditory  nerve  ;  7t.i>,  canal  for  the  nerve  to  the  ampulla 
of  the  posterior  semicircular  canal;  s.s.c,  ampullary  end  of  the  superior  canal  ; 
/,  ampullary  opening  of  the  posterior  cannl  ;  c,  common  aperture  of  the  superior  and 
posterior  canals  ;  e.s.c,  ampullary,  and  e'.s.c,  non-ampullary  end  of  the  external 
canal  ;  s.t.c,  scala  tympani  cochleae  ;  y.r,  fenestra  rotunda,  closed  by  its  membrane  ; 
a.  F,  aqueduct  of  Fallopius. 


labyrinth  (see  Fig.  1 1 9).  Within  the  bony  labyrinth  lies  a  membranous 
tube  of  corresponding  shape  called  the  membranous  labyrinth  (see  Fig.  118), 
which  does  not  entirely  fill  the  cavity  in  the  bone,  but  leaves  a  space  which 
is  filled  with  a  fluid  called  the  perilymph. 

The  mernbranous  labyrinth  is  likewise  filled  with  fluid,  which  is  termed 
the  endolymph.     The  parts  of  the  membranous  labyrinth  (see  Fig.  118)  are 


The  Senses. 


259 


the  utricle,  the  three  semicircular  canals^  the  saccule,  and  the  cochlea ;  and 
those  of  the  osseous  labyrinth  are  termed  the  vesiibule,  the  semicircular 
canals,  and  the  cochlea,  of  which  the  vestibule  accommodates  the  saccule 
and  utricle,  and  the  semicircular  canals  and  cochlea  the  correspondingly 
named  membranous  parts. 

The  vestibule  is  the  central  chamber  of  the  labyrinth,  and  communicates 
in  front  with  the  cochlea  and  behind  with  the  semicircular  canals.  Its 
outer  wall,  that  next  the  tympanum,  contains  the  fenestra  ovalis  and 
rotundis  mentioned  above. 

The  osseous  cochlea  is  a  gradually  tapering  tube  wound  in  a  spiral  of 


\jp.s.o. 


Fig.    118. — Plan    of  the   right   membranous 
labyrinth  viewed  from  the  mesial  aspect. 


utricle,  with  its  macula  and  the  three 
semicircular  canals  with  their  ampullae  ; 
J.,  saccule  ;  ag.v.,  aquseductus  vestibuli ; 
s.e-.,  saccus  endolj'mphaticus ;  c.r., 
canalis  reuniens  ;  c.c,  canal  of  the 
cochlea. 


Fig.  119. — View  of  the  interior  of  the  left 
osseous  labyrinth. 
The  bony  wall  of  the  labj-rinth  is  removed 
superiorly  and  externally.  i,  fovea 
hemi-elliptica  ;  2,  fovea  hemlsphasrica  ; 
3,  common  opening  of  the  superior  and 
posterior  semicircular  canals  ;  4,  open- 
ing of  the  aqueduct  of  the  vestibule  ; 
5,  the  superior,  6,  the  posterior,  and, 
7,  the  external  semicircular  canals ;  8, 
spiral  tube  of  the  cochlea  ;  9,  scala 
tympani  ;  10,  scala  vestibuli. 


two  turns  and  three-quarters  rovmd  a  slender  central  pillar  of  bone,  called 
the  modiolus,  from  which  a  spirally  wound  lamina  projects  inwards,  dividing 
the  tube  partially  into  two  compartments.  The  membranous  labyrinth  lies 
within  this  spiral  cavity,  and  between  the  membrane  and  the  bony  wall  a 
space  exists  filled  with  perilymph.  The  membranous  cochlea  is  divided  into 
three  distinct  tubes  (see  Fig.  120)  by  two  membranes.  One  of  these  mem- 
branes (the  basilar  membrane)  stretches  from  the  spiral  lamina  of  bone 
mentioned  above  to  the  opposite  wall  of  the  bony  cochlea,  thus  forming 
two  divisions  in  the  tube,  called  the  scala  tympani  (beneath)  and  scala 
vestibuli  (above).  The  second  membrane  {Reissner's  mcfnbranc)  meets  the 
basilar  membrane  at  an  angle,  and  shuts  off  a  small  spiral  chamber  from  the 
scala  vestibuli,  which  is  called  the  canal  of  the  cochlea  (see  Figs.  120,  121). 
It  is  within  this  canal  of  the  cochlea  that  the  organ  of  Corti  is  placed, 
resting  upon  the  basilar  membrane  (see  Fig.  121).     The  terminations  of  the 


26o 


Elementary  Physiology. 


Fig.  I20.— Vertical  section  of  the  cochlea  of  a  calf.     CKolliker.") 


Fig.  121.— Vertical  section  of  the  first  turn  of  the  human  cochlea.  (G.  Retzius.) 
s.v.  scala  vestibuli ;  s.t.,  scala  tympani ;  D.C,  canal  of  the  cochlea;  sp.l,  spiral 
'lamina  ;  n,  nerve  fibres  ;  l.sp.,  spiral  ligament ;  .y^r.t;.  stria  vascularis  ;  ssp  spiral 
eroove  •  R.  section  of  Reissner's  membrane  ;  I,  limbus  laminae  spiralis  M.t.,  mem- 
brana  tectoria  ;  t.C,  tunnel  of  Corti ;  b.m.,  basilar  membrane  ;  h.i.,  h.e.,  mternal 
and  external  hair  cells. 


The  Senses.  261 

nerve  fibres  of  the  auditory  nerve  lie  within  this  organ  of  Corti.  The  fibres 
course  out  in  a  radiating  manner  from  the  central  modiolus  to  be  dis- 
tributed to  the  organ  of  Corti  all  along  the  length  of  the  cochlea. 

A  complete  description  of  the  organ  of  Corti,  which  is  very  complex  in 
its  structure,  cannot  here  be  given ;  sufiice  it  to  say,  that  it  consists  of  modified 
epithelial  cells  arraiiged  in  rows  (see  Fig.  121),  and  having  an  appearance 
of  the  general  character  of  those  found  at  the  peripheral  distribution  of  the 
nerves  of  special  sense. ^  Certain  of  these  cells  are  furnished  with  hair-like 
processes  (auditory  hairs),  which  project  into  the  endolymph  bathing  the 
cells,  and  hence  are  brushed  upon  and  easily  affected  by  any  movement  of 
that  fluid.  The  hair  cells  are  placed  in  two  columns,  respectively  internal 
and  external,  to  two  rows  of  cells  called  the  rods  of  Corti,  which  incline 
towards  each  other  and  form  the  structure  known  as  the  tunnel  of  Corti. 
There  is  only  one  row  of  internal  hair  cells,  but  three  or  four  rows  are 
present  in  the  external  column.  Over  the  organ  of  Corti  there  lies  a  pad  of 
soft  fibrillar  tissue,  which  is  called  the  me?Jibrana  tectoria ;  during  life  this 
probably  lies  upon  the  hair  cells  and  forms  a  kind  of  damper  to  prevent 
excessive  disturbance  of  the  auditory  hairs. 

We  are  now  in  a  position  to  follow  out  the  chain  of  changes 
which  take  place  between  the  arrival  of  a  sound  wave  at  the 
membrana  tympani  and  the  arrival  of  the  modified  disturbance 
which  it  gives  rise  to  at  the  organ  of  Corti. 

A  sound  is  due  to  disturbance  propagated  through  the  air  in  the  form  of 
waves  of  rarefaction  and  compression.  As  the  waves  of  rarefaction  and 
compression  reach  the  membrana  tympani  this  is  alternately  moved  out 
and  in  by  them  at  the  same  rate.  Thus  the  membrane  is  set  vibrating  at 
the  same  rate  as  that  at  which  the  sound  waves  are  produced.  For  a  low- 
pitched  note  the  sound  waves  are  produced  more  slowly,  there  is  a  smaller 
number  of  vibrations  per  second,  and  for  a  high-pitched  note  the  vibra- 
tions are  more  rapid.  These  variations  in  rate  are  faithfully  copied  by 
the  movements  of  the  membrana  tympani.  Also,  the  louder  the  note,  the 
more  excessive  are  the  variations  of  rarefaction  and  compression,  and  so 
correspondingly  more  extensive  are  the  movements  of  the  membrane.  The 
movements  of  the  membrana  tympani  are  communicated  to  the  chain  of 
ossicles  in  the  middle  ear,  which  move  in  unison  with  it.  This  movement 
of  the  chain  of  ossicles  is  not  a  vibration  propagated  through  the  substance 
of  the  ossicles,  but  a  movement  of  each  ossicle  as  a  whole,  the  three  bones 
forming  a  system  of  levers.  The  length  of  the  arms  of  the  levers  is  so 
adjusted  that  the  extent  of  movement  is  diminished  about  one-third,  so  that 
the  excursion  of  the  stapes  is  only  about  two-thirds  of  that  of  the  membrana 
tympani.    There  is  also  an  arrangement  at  the  articulation  between  malleus 

^  Compare  the  olfactory  and  gustatory  cells  already  described,  and  also 
the  rods  and  cones  of  the  retina,  with  the  hair  cells  of  Corti's  organ. 


262  Elementary  Physiology. 

and  incus,  which  prevents  any  excessive;  movement  of  the  membrane,  such 
as  would  be  caused  by  the  sound  of  an  explosion,  being  communicated  to 
the  inner  ear  and  damaging  the  delicate  structures  there.  ^ 

The  movements  of  the  stapes  are  communicated  to  the  membrane 
covering  the  fenestra  ovalis  at  its  base.  It  will  be  remembered  that  the 
internal  ear  is  completely  filled  with  fluid,  which  is  enclosed  in  a  bony  wall, 
except  at  two  places,  viz.  the  fenestra  ovalis  and  the  fenestra  rotundis,  where 
the  bone  is  absent  and  the  perilymph  is  separated  from  the  tympanic  cavity 
only  by  membrane.  Now,  when  the  fenestra  ovalis  membrane  is  pushed  in 
by  an  inward  movement  of  the  stapes,  the  fenestra  rotundis  must  be  bulged 
out  towards  the  tympanum,  because  the  fluid  in  the  internal  ear  is  incom- 
pressible, and,  Tjice  versa,  when  the  membrane  covering  the  fenestra  ovalis  is 
pulled  outward  by  the  stapes  the  membrane  covering  the  fenestra  rotundis 
must  move  inwards.  It  is  thus  easy  to  see  why  there  must  be  two  movable 
partitions  between  tympanum  and  labyrinth,  for  otherwise  the  vibration  of 
membrana  tympani  and  ossicles  could  not  be  communicated  to  the  incom- 
pressible fluids  of  the  labyrinth. 

As  the  membranes  covering  the  fenestra  ovalis  and  fenestra  rotundis  thus 
move  inward  and  outward  in  time  to  the  swingings  of  the  membrana 
tympani  and  ossicles,  there  is  a  surging  in  equal  rhythm  caused  of  the 
perilymph.  This  vibratory  swinging  of  the  perilymph  is  in  turn  com- 
municated to  the  endolymph,  and  passes  round  the  chambers  of  the  cochlea, 
up  the  scala  vestibuli,  which  lies  nearer  the  fenestra  ovalis,  to  the  scala 
tampani  which  communicates  at  the  head  of  the  spiral  with  the  scala 
vestibuli,  and  down  this  towards  the  fenestra  rotundis.  In  its  passage, 
along  the  cochlear  chambers,  this  fluid  vibration  affects  the  fluid  in  the  canal 
of  the  cochlea,  disturbs  the  auditory  hairs,  and  institutes  changes  in  the 
cells  of  the  organ  of  Corti,  which  are  translated  somehow  into  nerve 
impulses,  and  these,  finally,  travel  up  to  the  auditory  centres  in  the  brain  and 
there  awaken  auditory  sensations. 

Deafness  may  arise  from  an  incompetence  at  any  part  of  this 
complicated  system.  It  may  be  due  to  blocking  of  the  external 
auditory  meatus  (for  example,  by  impacted  wax),  preventing 
vibration  of  the  tympanic  membrane  ;  it  may  be  due  to  serious 
injury  of  the  membrane  itself,  although  a  slight  perforation  is 
often  present  in  persons  with  normal  hearing  ;  it  may  be  due  to 
permanent  closure  of  the  Eustachian  tube,  in  which  case  there 
is  no  longer  equal  pressure  on  the  two  sides  of  the  membrane, 
the  middle  ear  becomes  charged  with  exuded  fluid,  and  vibra- 
tion of  the  membrane  and  ossicles  becomes  impossible  ;  it  may 

\  For  a  complete  account  of  the  articulations  and  movements  of  the 
auditory  ossicles,  see  Quain's  "Anatomy,"  vol.  iii.  pt.  iii.  p,  95. 


The  Senses.  263 

be  due,  though  this  form  is  rare,  to  disease  in  the  auditory 
ossicles;  it  may  be  due  to  defects  in  the  internal  ear;  and, 
finally,  it  may  arise  from  disease,  injury,  or  congenital  defect  in 
the  cerebral  cortex  (see  Fig.  112). 

The  semicircular  amals  (see  Figs.  118,  119),  although  they 
are  anatomically  so  closely  associated  in  the  internal  ear  with  the 
auditory  apparatus,  have  probably  no  connection  with  hearing, 
but  are  concerned  with  the  sense  of  equilibrium  and  with  the 
position  (or  rather  changes  in  position)  of  the  head  in  space. 
The  canals  are  elliptical  in  shape,  each  forming  about  two- 
thirds  of  an  ellipse,  and  the  osseous  canals  open  into  the 
vestibule  hy  Jive  openings,  for  two  join  together  at  one  end. 
The  membranous  canals  are  much  more  slender  than  the  bony 
canals  within  which  they  lie,  and  the  space  between  is  filled 
with  perilymph.  Each  membranous  semicircular  canal  has  a 
swelling  near  one  end  termed  an  ampulla,  and  on  the  inner 
surface  of  this  there  is  a  ridge  called  the  crista  acustica,  upon 
which  an  epithelium  containing  cells  with  hair-like  processes  is 
placed.  This  epithelium  is  provided  with  nerve  fibres  derived 
from  a  portion  of  the  auditory  nerve,  termed  the  vestibular 
nerve.  Any  motion  of  the  fluid  (endolymph)  within  the  canal 
affects  these  hair-like  processes,  and  gives  rise  to  nerve  impulses. 
The  three  canals  are  set  in  three  planes  at  right  angles  to  one 
another,  and  hence  motion  of  the  head  cannot  take  place  in  any 
direction  whatever  without  causing  a  movement  of  the  endo- 
lymph certainly  in  one,  and  usually  in  two,  of  the  three  canals. 
The  movements  of  the  head  thus  give  rise  to  sensations  which 
supply  a  means  of  judging  of  the  movements  and  aid  in  main- 
taining equilibrium.  Disease  of  the  canals,  or  experimental 
injury  to  them,  causes  injury  to  the  sense  of  equilibrium,  failure 
in  balancing  the  body,  and  movements  of  rotation  of  the  head 
corresponding  to  that  canal  which  is  injured. 

Sight.  ^ 
In  treating  of  vision  it  will  be  well   in  the  first  place   to 
consider  the  structure  of  the  eye ;  next,  its  action  as  an  optical 

^  The  student  is  strongly  recommended  to  accompany  this  description 
by  a  dissection  of  an  ox  or  pig's  eye. 


264 


Elementary  Physiology. 


instniment ;  and,  lastly,  the  nature  of  the  peripheral  nervous 
structures  which  are  affected  by  the  light.  The  eyeball  lies  in 
the  forepart  of  the  orbital  cavity,  where  it  is  supported  upon 
a  soft  circular  cushion  or  pad  of  fat,  and  is  surrounded  by  six 
muscles,  by  means  of  which  it  can  be  turned  to  a  certain 
extent  in  various  directions. 


Fig.  122. — Section  through  the  orbit  and  its  contents. 
a,  frontal  bone  ;  b,  superior  maxillary  bone  ;   r,  eyebrow  ;  d,  eyelids  ;  e,  conjunctiva ; 
f,  the  muscle  which  raises  the  upper  lid  ;  ^and  g  ,  recti  muscles  ;  h,  inferior  oblique 
muscle  cut  across  ;    i,  optic  nerve;   2,  cornea  ;   2',  sclerotic  ;   3,  aqueous  chamber; 
4,  crystalline  lens  ;   5,  vitreous  chamber. 


Four  of  these  muscles  run  straight  forward  from  the  posterior  part  of 
the  orbit  to  be  inserted  to  the  eyeball  in  front.  They  are  hence  called 
the  recti  muscles,  and  according  to  position  are  known  as  the  superior, 
inferior,  external,  and  internal  rectus,  respectively.  Their  action  corre- 
sponds to  these  names ;  thus,  the  superior  rectus  raises,  while  the  inferior 
depresses,  the  front  part  of  the  eyeball ;  the  external  rectus  turns  it  outwards 
in  front,  and  the  internal  turns  it  inwards. 

The  other  two  muscles  are  called  the  superior  and  inferior  oblique,  from 
their  mode  of  attachment.  The  superior  oblique  muscle  is  attached  to  the 
posterior  part  of  the  orbit  on  the  inner  side,  and  passing  forward  ends  in 
a  tendon,  which  loops  round  a  kind  of  pulley  of  fibres  attached  to  the 
frontal  bone,  and  then  passes  backward  and  outward  to  become  fixed  to 
the  eyeball  at  its  outer  and  back  portion.  When  this  muscle  contracts,  it 
turns  the  front  of  the  eyeball  obliquely  outward  and   downward.     The 


The  Senses.  265 

inferior  oblique  muscle  arises  from  the  lower  and  front  part  of  the  orbit 
on  the  nasal  side,  and  passes  obliquely  backwards  and  outwards  to  be 
inserted  on  the  posterior  and  outer  part  of  the  eyeball  ;  by  its  contraction, 
the  front  of  the  eyeball  is  moved  upward  and  outward.  The  muscles  of 
the  two  eyeballs  work  together  in  most  perfect  co-ordination,  so  that  both 
eyes  are  kept  trained  upon  the  same  object  at  the  same  time,  and  by  no 
effort  of  the  will  can  one  eyeball  be  moved  without  a  corresponding  move- 
ment of  the  other.  This  is  necessary  in  order  that  images  of  objects  may 
be  cast  upon  corresponding  points  of  the  sensitive  screens  at  the  backs  of 
the  two  eyeballs  ;  otherwise,  double  vision  is  the  result.  This  may  be 
shown  by  artificially  changing  the  direction  of  one  eyeball  by  pressure  upon 
it  with  the  forefinger,  when  objects  are  at  once  seen  double.  In  this 
complex  co-ordination  of  the  oculi-motor  muscles,  anatomically  correspond- 
ing muscles  in  the  two  eyes  do  not  always  work  together ;  for  example, 
when  the  eyes  are  moved  from  side  to  side  the  external  rectus  of  one  eye 
contracts  at  the  same  time  as  the  internal  rectus  of  the  other.  In  this 
manner  the  internal  rectus  of  one  eye  and  the  external  of  the  other  are,  so 
to  speak,  yoked  together  ;  on  the  other  hand,  the  two  superior  recti  muscles 
work  together,  and  so  do  the  two  inferior  recti  muscles. 

^Vhen  the  external  surface  of  an  eyeball  is.  examined  it  is 
seen  to  be  composed  of  segments  of  two  spheres.^  One  of 
these  spherical  segments  forms  the  middle  portion  of  the  front 
of  the  eyeball.  It  is  by  far  the  smaller  of  the  two  segments, 
forming  only  about  one-fifth  of  the  antero-posterior  circum- 
ference of  the  eyeball.  The  smaller  segment  is  called  the 
cornea^  and  the  larger  segment  the  sclerotic.  The  cornea  is 
transparent,  and  allows  light  to  enter  the  eye ;  while  the 
sclerotic  is  opaque,  and  allows  no  passage  to  the  light. 

The  external  surface  of  the  cornea  and  the  anterior  portion  of  the 
selerotic  are  covered  by  a  delicate  mucous  membrane  called  the  conjunctiva, 
which  is  also  reflected  all  round  over  the  internal  surfaces  of  the  eyelids. 
The  conjunctiva  is  plentifully  supplied  with  sensory  nerve  endings  which 
make  it  very  sensitive  to  any  irritant,  and  is  kept  moist  by  the  secretion  of 
a  special  gland  lying  in  the  orbit  called  the  lacrymal  gland.  When  for  any 
reason  the  secretion  of  this  gland  becomes  too  copious,  the  space  between 
the  eyeball  and  eyelids  fills  up  and  overflows,  so  giving  rise  to  tears. 

The  cornea,  to  superficial  examination,  appears  to  be  of 
different  colours  in  different  persons,  but  it  is  really  quite 
transparent  and  colourless,  as  may  be  seen  in  dissecting  an  eye 

'  This  may  also  be  appreciated  by  closing  the  eye,  placing  a  forefinger 
over  the  upper  eyelid  and  moving  it  about,  when  the  prominence  of  the 
anterior  segment  can  be  distinctly  felt. 


266  Elementary  Physiology. 

or  by  looking  at  the  cornea  of  another  person's  eye  from  the 
the  side.  The  colour  is  due  to  a  pigmented  screen,  lying 
inside  the  eyeball  behind  the  cornea,  called  the  iris.  In  the 
middle  of  the  iris  there  is  a  circular  opening,  through  which 
the  light  enters  the  eye,  called  the  pupil.  No  matter  what  be 
the  colour  of  the  eye,  this  central  opening  is  always  black 
(except  in  albinos),^  because  all  the  posterior  part  of  the 
internal  surface  of  the  eyeball  is  lined  by  a  black  pigmented 
coat  called  the  choroid  coat,  which  reflects  back  no  light,  and 
so  gives  a  black  colour  to  the  pupil. 

At  the  back  of  the  eyeball,  a  little  to  the  nasal  side  of  the 
pole  of  the  posterior  hemisphere,  a  thick  nerve  trunk,  that  of 
the  optic  nerve,  pierces  the  sclerotic  coat  and  enters  the  eyeball. 
Its  branches  and  endings  are  spread  out  over  the  posterior  inner 
surface  of  the  eyeball  underlying  the  innermost  of  the  coats  of 
the  eyeball,  which  is  known  as  the  retina  and  contains  the 
elements  which  are  sensitive  to  the  light.  The  blood-vessels 
for  the  supply  of  the  eyeball  enter  it  along  with  the  optic,  nerve, 
and  ramify  in  the  choroid  coat.  The  structures  lying  within 
the  eyeball  may  be  seen  by  making  a  section  from  front  to 
back  with  a  sharp  razor,  and  by  dividing  the  sclerotic  with  sharp 
scissors  circularly,  parallel  to,  and  somewhat  behind  the  margin 
of  the  cornea.^  The  structures  displayed  in  an  antero-posterior 
section  are  figured  in  the  accompanying  illustration  (Fig.  123). 
Behind  the  cornea  lies  the  anterior  cha??iber  of  the  eye,  which  is 
filled  with  a  thin  watery  fluid  called  the  aqueous  humour. 

The  anterior  chamber  is  bounded  posteriorly  by  the 
crystalliiie  lens,  and  by  the  suspensory  ligament  which  attaches 
the  lens  all  round  to  processes  (ciliary  processes)  which  arise 
from  the  anterior  margin  of  the  choroid  coat. 

The  crystalline  lens  (see  Fig.  123)  is  a  solid  body  composed 
of  perfectly  transparent  fibres.  It  is  a  convex  lens  of  which 
the  convexity,  and  hence  the  focal  length,  can  be  altered,  and 
its  purpose  is  to  focus  images,  on  the  posterior  inner  surface 
of  the  eyeball,  of  objects  situated  at  variable  distances  in  the 

^  In  albinos  this  black  pigment  is  absent  from  the  choroid  coat,  and 
hence  the  pupil  looks  pink,  because  of  the  pink  colour  of  the  light  reflected 
back  through  the  pupil  from  the  blood-vessels  lying  in  the  choroid, 

^  See  Appendix. 


The  Senses. 


267 


field    of  view  according  as  the  attention  is  directed  to  these 
various  objects. 

Behind  the  lens  lies  the  posterior  chamber  of  the  eye,  which 


Fig.  123. — View  of  the  human  ej'e,  divided  horizontally  through  the  middle. 
I,  conjunctiva;    2,    cornea;    3,    sclerotic;    4,  sheath  of  the   optic  nerve;    5,  choroid; 
6,  ciliary  processes  ;    7,  iris ;    8,  pupil  ;    9,  retina  ;     10,  anterior  limit  of  the  retina  ; 
II,    crj'stalline    lens  ;     12,    suspensory  ligament  ;    13,    ciliary  muscle  ;     14,    anterior 
chamber;    15,  posterior  chamber  ;    i6,  yellow  spot ;  17,  blind  spot. 


is  completely  filled  by  a  clear  jelly-like  mass  called  the  vitreous 
humour. 

The  iris  lies  in  the  anterior  chamber  in  front  of  the  lens, 
and  its  variable  central  apertm-e  (the  pupil)  lies  opposite  the 
central  portion  of  the  lens. 

There  is  a  circular  muscle  within  the  eyeball  called  the 


268  Elementary  Physiology. 

ciliary  muscle  lying  at  the  junction  between  cornea  and 
sclerotic.  Some  of  the  fibres  of  this  muscle  are  disposed 
circularly,  running  parallel  to  the  junction  of  cornea  and 
sclerotic ;  others  run  longitudinally,  and  are  attached  in  front 
to  the  corneo-sclerotic  junction  and  behind  to  the  anterior 
margin  of  the  choroid  coat.  When  the  longitudinal  fibres 
contract  they  pull  forward  the  anterior  margin  of  the  choroid 
coat,  and  so  slacken  the  suspensory  ligament  of  the  lens 
which  then,  by  virtue  of  its  elasticity,  takes  a  more  convex 
form.  Since  the  posterior  surface  of  the  lens  must  conform 
to  the  shape  of  the  vitreous  humour  behind  it,  the  alteration 
in  shape  takes  place  chiefly  at  the  anterior  surface.  The 
purpose  of  this  alteration  in  shape  of  the  lens  brought  about 
by  the  action  of  the  ciliary  muscle  is  to  focus  objects  upon 
the  retina.  When  a  near  object  is  examined  the  lens  is  made 
more  convex;  when  a  distant  object  is  looked  at  it  becomes 
flatter.^  The  circular  fibres  of  the  ciliary  muscle  aid  the 
longitudinal  in  pulling  the  choroid  forward. 

In  the  region  of  the  eyeball  behind  the  ciliary  muscle  and 
external  to  the  viteous  humour  there  are  three  coats  in  the 
wall  of  the  eyeball.  Of  these,  the  inner  is  termed  the  retina, 
it  contains  the  elements  which  are  sensitive  to  the  light;  the 
middle  coat  is  called  the  choroid,  it  contains  cells  which  form 
black  pigment,  and  in  it  run  the  blood-vessels  of  the  eyeball ; 
the  external  coat  is  the  sclerotic,  which  is  composed  of  tough 
white  fibrous  tissue. 

The  eye  as  an  optical  instrument  bears  a  certain  resem- 
blance to  a  photographic  camera.  In  both  there  is  an  inverted 
image  formed  of  objects  in  the  external  world  upon  a  sensitive 
screen  placed  at  the  back ;  but  in  the  eye  the  image  gives  rise 
to  nerve  impulses  which  affect  the  consciousness,  while  in  the 
photographic  camera  the  effect  produced  by  the  light  rays  is 
made    obvious   by   future   chemical   manipulation.      In   both 

*  In  the  case  of  an  ordinary  convex  lens  of  glass,  when  the  object  is 
brought  nearer,  the  image  is  formed  farther  off  on  the  opposite  side  of  the 
lens.  But  this  would  not  be  suitable  in  the  eye,  in  which  the  screen  or  retina 
is  placed  at  a  constant  distance  ;  hence  the  lens  is  made  accommodati?ig, 
becoming  more  convex  when  the  object  is  nearer,  and  so  keeping  it  focussed 
upon  the  retina  by  bringing  the  rays  to  a  focus  in  a  shorter  distance. 


The  Senses,  269 

there  is  an  internal  black  lining,  the  purpose  of  which  is  to 
prevent  blurring  of  the  image  by  internal  reflection  of  the  light. 
In  both  the  images  are  focussed  upon  the  screen  at  the 
back;  but  in  the  eye  this  focussing  or  accominodatioji  is  effected 
by  a  change  in  the  focal  length  of  the  lens,  while  in  the  camera 
it  is  effected  by  changing  the  distance  of  the  lens  from  the 
screen.  Further,  in  the  eye  the  whole  chamber,  by  virtue  of 
its  shape,  acts  as  a  converging  lens  in  focussing  objects  upon 
the  retina,  and  the  lens  itself  is  chiefly  of  use  as  an  adjusting 
mechanism — a  kind  of  fine  adjustment.  Thus,  there  is  con- 
vergence caused  by  the  convex  anterior  surface  of  the  cornea, 
and  convergence  by  the  refracting  convex  mass  of  the  vitreous 
humour.  In  the  photographer's  camera,  on  the  other  hand, 
the  chamber  contains  only  air,  and  all  the  focussing  is  done 
by  means  of  the  lens.  In  both  the  amount  of  light  which 
is  allowed  to  enter  is  regulated  by  stops  or  diaphragms^ 
according  to  the  intensity  of  illumination  of  the  objects  of 
which  images  are  to  be  cast  upon  the  sensitive  screens. 

In  the  eye  the  adaptation  of  the  size  of  the  pupil,  which  forms  the 
diaphragm  of  the  eye,  does  not  take  place  instantaneously,  but  follows  some- 
what slowly  by  reflex  nervous  action  when  the  intensity  of  illumination  of 
the  retina  is  varied.  It  is  for  this  reason  that  we  are  unable  to  see  for  a 
few  moments  when  we  suddenly  leave  a  brilliantly  illuminated  room  and 
pass  out  into  a  darkly  lit  space,  until  the  pupil  becomes  widened  to  suit  the 
dimmer  light.  Conversely,  when  we  enter  from  comparative  darkness  to  a 
place  where  there  is  a  bright  light  we  are  at  first  dazzled  by  the  excessive 
amount  of  light  entering  the  eye,  until  the  pupil  contracts  and  relieves  the 
sensitive  retina.  These  changes  are  occasioned  by  two  sets  of  muscle 
fibres  in  the  iris  ;  one  set  being  arranged  circularly  around  the  pupil  and 
the  other  disposed  radially. 

The  retina  or  sensitive  screen  upon  which  inverted  images 
of  objects  are  focussed  by  the  optical  action  of  the  refractive 
substances  of  the  eyeball  consists  of  eight  different  layers 
which  are  diagrammatically  shown  in  the  accompanying  figure 
(Fig.  124). 

Of  these  layers  that  called  the  layer  of  rods  and  cones  is 
the   one   which   is    primarily  affected  by  the   light. ^      It   lies 

^  This  is  shown  by  the  fact  that  this  is  the  only  layer  present  in  the 
fovea  centralis  {vide  infra),  which  is  the  most  sensitive  part  of  the  retina. 


2/0 


Elementary  Physiology. 


farthest  removed  from  the  entering  Hght  next  the  choroid  coat, 
and  is  bound  by  certain  cells  containing  pigment  called  the 
pigment  cells. 

The  retina  is  not  equally  sensitive  in  all  parts.  Where  the  optic 


Fig.  124. — Diagrammatic  section  of  the  human  retina.     (Schuhze.) 

nerve  enters,  a  little  to  the  nasal  side  of  the  pole  of  the  posterior 
hemisphere,  the  retinal  elements  are  absent,  and  the  branches 
of  the  optic  nerve  spread  out  in  all  directions  to  form  a  net- 
work.    This  region  is  called  the   "  blind  spot^'  and  is  quite 


TJie  Senses. 


2/1 


insensitive ;  so  that  when  the  image  of  an  object  falls  upon  it 
no  visual  sensation  is  produced.  About  a  tenth  of  an  inch  to 
the  temporal  side  of  this  there  is  a  small  yellow  spot  called  the 
macula  lutea^  of  slightly  elliptical  shape  (see  Fig.   125),  which 


Fig.  125. — View  of  the  posterior  part  of  the  retina. 

A. — The   posterior  half  of  the  retina   of  the  left  ej-e,  viewed  from  before.     (Henle.) 

Twice  the  natural  size. 

s,  cut  edge  of  the  sclerotic  ;  ch,  choroid  ;  r,  retina  :  in  the  interior  at  the  middle  the 
macula  luteawith  the  depression  of  the  fovea  centralis  is  represented  by  a  slight  oval 
shade  ;  towards  the  left  side  the  light  spot  indicates  the  colliculus  or  eminence  at 
the  entrance  of  the  optic  nerve,  from  the  centre  of  which  the  arteria  centralis  is  seen 
sending  its  branches  into  the  retina,  leaving  the  pait  occupied  by  the  macula  com- 
paratively free. 


has  in  its  central  part  a  slight  depression  called  the  fovea 
centralis.  The  fovea  centralis  lies  at  the  opposite  pole  of  the 
eyeball  from  the  pupil,  and  is  the  part  of  the  retina  upon 
which  vision  is  most  acute.  When  the  attention  is  directed 
to  an  object,  the  eyeballs  are  so  turned  that  the  image  of  it  is 
focussed  upon  the  fovea  centralis  and  the  parts  of  the  macula 
lutea  adjoining.  Objects,  of  which  the  images  are  cast  upon 
other  parts  of  the  retina,  are  only  seen  very  indistinctly,  and 
are  more  indistinct  as  their  images  fall  more  distant  from  the 
fovea  centralis. 

The  existence  of  the  blind  spot  may  be  easily  demon- 
strated by  making  two  round  dots  on  a  piece  of  white  paper, 
each  about  one-tenth  of  an  inch  in  diameter,  and  about  two 


2/2  Elementary  Physiology. 

inches  apart,  and  then  observing  these  two  spots  with  each  eye 
in  succession,  the  other  eye  being  closed  in  each  case. 

While  observing  with  the  right  eye  look  steadily  at  the  left- 
hand  dot  and  move  the  sheet  of  paper  nearer  and  farther 
alternately  from  the  open  eye.  When  the  paper  is  near  the 
eye  both  dots  are  easily  visible,  but  at  a  certain  distance  the 
right-hand  dot  completely  disappears  and  does  not  re-appear 
until  the  paper  has  been  moved  considerably  farther  from  the 
eye.  A  similar  result  cannot  be  obtained  in  the  case  of  the 
left-hand  dot  by  looking  at  the  right-hand  dot  with  the  right 
eye.  A  disappearance  of  the  left  dot  can  similarly  be  obtained 
in  the  left  eye  when  the  right  dot  is  focussed  upon  the  fovea, 
but  not  of  the  right  dot  when  the  left  dot  is  so  focussed. 
Remembering  that  the  light  rays  cross  in  the  eye  before 
reaching  the  retina,  and  hence  that  the  position  of  images  is 
inverted,  a  little  consideration  will  show  that  these  experi- 
ments demonstrate  that  the  blind  spot  lies  to  the  nasal  side  of 
the  fovea  centralis. 

The  visual  sensation  outlasts  in  time  the  stimulation  of  the 
retina  by  the  light.  The  sensation  does  not  quite  disappear 
until  the  lapse  of  about  one-tenth  of  a  second  after  the  stimu- 
lation. Hence,  when  a  light  flashes  more  than  ten  times  in  a 
second,  the  flashes  fuse  into  one  another,  and  the  sensation  pro- 
duced is  a  continuous  light,  but  unsteady  and  flickering.  If  the 
flashes  succeed  one  another  at  a  rate  of  fifty  to  sixty  per  second 
the  flickering  sensation  disappears  and  the  flashes  are  fused 
into  a  steady  light.  This  continuance  of  sensation  after  stimu- 
lation is  the  explanation  of  such  famihar  phenomena  as  the 
circle  of  fire  caused  by  rotating  a  stick  with  a  glowing  end, 
the  fusion  of  colours  in  the  rotating  colour  top,  and  the  fusion 
of  the  spokes  of  a  rapidly  rotating  wheel. 

We  can  distinguish  many  hues  and  shades  of  colour  in  the 
objects  which  we  see,  but  all  these  varied  colours  only  awaken 
three  kinds  of  sensation.^     Coloured  light  of  any  given  wave- 

*  Rival  theories  have  been  advanced  to  explain  the  phenomena  of  colour 
vision ;  these  theories  are  both  exceedingly  artificial  and  insufficient,  and  so 
an  attempt  has  been  made  below  to  give  an  outline  of  a  few  of  the  prmcipal 
facts. 


The  Senses.  273 

length  arouses  all  of  these  three  sensations,  but  in  very  varying 
degree,  and  the  sensation  produced  as  a  resultant  varies  with 
the  comparative  intensity  of  the  three  component  sensations. 
The  three  primary  sensations  are  each  most  strongly  produced 
by  red,  green,  and  violet  light  respectively,  and  these  are 
hence  spoken  of  as  primary  colours.  However,  red  light  only 
awakens  the  red  sensation  in  a  preponderating  degree ;  the 
other  sensations  are  also  produced,  only  much  more  feebly. 
So  any  tint  is  produced  by  certain  fixed  degrees  of  intensity  of 
simultaneous  stimulation  of  the  three  kinds,  and  any  variation 
in  the  relative  strengths  of  the  three  component  stimuli  will 
cause  a  corresponding  variation  in  the  tint.  This  can  be 
experimentally  shown  by  fusing  together,  so  that  they  affect  the 
same  portion  of  the  retina,  coloured  lights  in  varying  intensity, 
as,  for  example,  by  casting  them  on  the  same  surface,  or  by 
spinning  them  on  a  colour  top. 

The  sensation  produced  by  light  of  any  given  physical 
composition  varies  greatly  with  the  condition  at  the  time  being 
of  the  visual  apparatus ;  not  only  so,  but  the  condition  of  the 
part  of  the  retina  immediately  around  the  area  stimulated  has  a 
profound  effect  on  the  sensation  evoked. 

The  visual  apparatus  is  easily  fatigued,  and  while  in  a 
fatigued  condition  it  reacts  less  easily  than  when  in  a  fresh 
condition;  again,  it  can  be  fatigued  for  one  colom',  and  yet 
react  normally  to  a  colour  for  which  it  has  not  been  fatigued. 

If  a  bright  white  spot  be  looked  upon  for  10  to  20  seconds, 
and  then  the  eyes  be  turned  to  a  dull  uniform  grey  background, 
or  be  closed,  an  image  of  the  spot  in  black  or  duller  colour, 
which  is  called  a  negative  after-image^  is  seen  on  a  brighter  back- 
ground. If  a  coloured  spot  be  employed  in  the  first  case,  an  after- 
image of  the  complementary  colour  is  seen — that  is,  an  image 
which  if  used  with  the  original  would  produce  white  as  a  result. 
Here  the  colour  of  light  is  changed  by  previous  actions  carried 
out  upon  the  part  of  the  visual  apparatus  causing  the  sensa- 
tion— in  other  words,  the  sensation  produced  is  shown  by  the 
experiment  to  vary  altogether  with  the  condition  at  the  moment 
of  the  visual  mechanism  stimulated.  Again,  if  a  colour  be 
viewed  upon  a  background    of  its    complementary   colour,   it 


2/4  Elementary  Physiology. 

appears  much  brighter  from  what  is  called  simultafieoiLS  contrast^ 
showing  that  the  sensation  is  influenced  by  the  condition  of 
neighbouring  parts  of  the  visual  apparatus. 

The  result  of  simultaneous  contrast  is  beautifully  shown  by 
the  experiment  of  Mayer,  which  consists  in  covering  with  tissue- 
paper  a  small  piece  of  grey  paper  placed  upon  a  coloured  paper 
surface;  the  grey  paper  then  appears  coloured  with  a  tint 
complementary  to  that  of  the  colour  upon  which  it  is  placed. 

In  some  persons,  who  are  said  to  be  colour  blind ^  the  per- 
ception of  colours  is  imperfect.  Such  persons  usually  have 
but  two  colour  sensations,  and  derive  all  their  sensations  of  tint 
from  fusion  of  these  two.  In  the  commonest  form  of  colour 
blindness,  red  and  green  appear  alike,  and  a  red  cherry  is  of 
the  same  colour  to  such  people  as  the  leaves  of  the  tree  on 
which  it  grows ;  in  other  persons,  the  middle  of  the  spectrum 
is  of  a  neutral  tint,  and  red  and  blue  gradually  fuse  into  each 
other.  Unfortunately,  such  persons  are  always  born  colour 
blind,  and  so  we  cannot  compare  sensations  with  them,  or 
else  important  facts  concerning  colour  sensation  might  be 
discovered. 

^  Successive  contrast  is  when  a  colour  is  viewed  after  its  complementary 
colour  ;  here  again  the  colour  is  brighter,  showing  in  a  converse  manner  the 
same  truth  as  after-images,  that  the  effect  depends  upon  the  immediately 
previous  history  of  the  part  stimulated  :  this  is  a  contrast  in  time,  while 
"  smiultanenus  contrast"  is  a  contrast  in  space. 


APPENDIX   OF   PRACTICAL   EXERCISES 

I .  Make  dissections  of  the  body  of  any  small  mammal,  such  as  a  rabbity 
guinea-pig,  white  rat,  or  kitten.^ 

Fasten  the  animal  upon  its  back,  make  an  incision  through  the  skin  in 
the  mid-line  in  front  all  the  way  down,  and  draw  the  skin  aside.  Cut 
through  the  abdominal  wall  in  the  mid-line  and  make  a  cross  cut  above, 
parallel  to  the  ribs  on  each  side. 

Examine  the  abdominal  viscera,  and  observe  their  position.  If  the 
animal  has  just  been  killed,  note  the  peristaltic  movements  of  the  intestine, 
and  start  a  contraction  at  any  point  by  touching  with  the  point  of  a  knife. 
Observe  the  long  pause  (latent  period)  before  the  contraction  commences. 
Examine  the  way  in  which  the  intestine  is  attached  to  the  abdominal  wall 
by  the  mesentery,  and,  if  the  animal  has  recently  been  fed  on  a  fatty  meal, 
look  for  the  lacteals  in  it,  filled  with  a  milky  fluid  (chyle).  Note  the  posi- 
tion of  the  various  viscera  as  described  in  the  text  (pp.  87-95).  Look  for 
the  pancreas  lying  in  the  loop  of  the  duodenum.  In  a  herbivorous  animal 
this  gland  is  diffuse  and  inconspicuous,  but  it  is  easily  found  in  the  carni- 
vora.  Pull  the  stomach  to  the  right,  and  observe  on  the  left  side  the  spleen 
and  its  attachments.  Look  also  for  the  opening  of  the  small  into  the  large 
intestine,  and  observe  the  caecum  and  vermiform  appendix,  which  are  large 
in  herbivora,  but  small  in  carnivora. 

Next  tie  two  ligatures  around  the  oesophagus,  a  short  distance  apart, 
and  cutting  between  the  two  ligatures,  and  then  through  the  mesentery,  re- 
move the  intestine  down  to  the  rectum,  where  two  other  ligatures  are  to  be 
tied  and  cut  between.  Cut  open  the  small  intestine,  and  observe  the  velvety 
appearance  of  the  inner  surface  caused  by  the  projecting  villi ;  look  at  it  with 
a  magnifying  glass  after  washing  it  with  water.  Similarly  examine  the 
inner  surfaces  of  the  stomach  and  large  intestine,  and  note  that  there  are  no 
villi  on  these  surfaces.  Note,  in  the  case  of  the  stomach,  the  thick  glandular 
mucous  membrane,  which  is  often  thrown  into  ruga:^  or  folds. 

After  the  removal  of  the  stomach  and  intestine,  observe  the  position  of 
the  kidneys,  and  the  small  suprarenal  bodies  lying  above  them,  and  of  the 

^  The  animal  may  be  most  painlessly  killed  by  placing  it  under  a  glass 
bell  jar  or  other  cover  with  a  piece  of  cotton  wool  soaked  in  chloroform. 
All  the  dissections  described  below  can  scarcely  be  made  at  one  time,  but 
can  be  made  at  intervals  as  opportunity  offers. 


276  Elementary  Physiology. 

liver.  Remove  the  liver  and  the  kidneys,  and  examine  them.  Note  the 
lobes  of  the  liver  and  the  position  of  the  gall-bladder.  Observe  the  impres- 
sions on  their  surfaces,  which  correspond  to  the  adjacent  viscera.  Observe 
how  the  vessels  enter  the  gland,  cut  into  it  and  examine  the  substance  of 
it.  Slice  open  one  of  the  kidneys  longitudinally,  compare  it  with  Fig.  98, 
p.  209,  and  read  the  description  of  its  naked-eye  appearance  there  given. 
Similarly  cut  open  one  of  the  suprarenal  glands,  and  the  spleen. 

Examine  the  abdominal  cavity  after  the  removal  of  these  organs  j  note 
the  smooth  lining,  and  search  for  the  aorta  and  inferior  vena  cava,  which 
will  be  found,  if  they  have  not  been  accidentally  removed  in  the  dissection, 
running  down  in  front  of  and  parallel  to  the  vertebral  column. 

Next  dissect  the  thorax,  commencing  by  removing  the  front  portion  of 
the  thoracic  wall.  In  a  small  animal,  the  ribs  can  be  cut  through  on  each 
side  with  a  strong  pair  of  scissors,  and  the  front  parts  of  the  ribs  and  the 
sternum  completely  removed.  When  the  thorax  is  opened  the  lungs 
collapse  and  occupy  a  comparatively  small  volume  ;  it  must  be  remembered 
thai  when  the  thorax  is  complete  they  are  distended,  and  occupy  the  greater 
part  of  the  cavity. 

Observe  the  position  of  the  heart  and  the  great  vessels  passing  to  and 
from  it  at  its  base  ;  note  the  pericardium  which  encloses  it,  and  then  cut 
through  this  and  expose  the  heart.  In  an  animal  which  has  just  been  killed, 
the  heart  may  often  be  made  to  contract  by  pricking  it  with  a  sharp-pointed 
instrument.  Look  at  the  roots  of  the  lungs  and  the  vessels  and  bronchi 
constituting  each.  Feel  the  lung  tissue,  which  gives  a  crepitant  sensation 
to  the  fingers.  Dissect  up  into  the  neck  so  as  to  expose  the  trachea  and 
larynx.  Find  the  oesophagus  behind  the  trachea,  and  trace  it  through  the 
thorax  to  the  point  at  which  it  passes  through  the  diaphragm.  Examine 
the  diaphragm  lying  between  the  thorax  and  abdomen,  and  note  the  shape 
of  the  central  tendon.  Try  to  find  the  places  where  the  inferior  vena  cava 
and  the  aorta  penetrate  it,  and  trace  these  vessels  up  to  the  heart.  Remove 
the  trachea  and  lungs,  tie  a  tube  into  the  trachea,  blow  air  into  it,  and  show 
that  the  lungs  become  distended  and  greatly  increased  in  volume.^  Re- 
move the  heart  and  the  roots  of  the  great  vessels  attached  to  it.  Examine 
its  structure  as  far  as  is  possible  with  a  heart  of  such  small  size  (see  pp. 
1 13-120), 

Next  remove  the  skin  from  the  lower  part  of  the  animal  and  dissect  one 
leg.  Turn  the  animal  back  upwards,  and  examine  the  structure  and  arrange- 
ment of  the  muscles  at  the  back  of  the  thigh  and  leg.  Separate  the  muscles 
on  the  back  of  the  leg  and  search  for  the  sciatic  nerve,  which  will  be  found 
lying  deep  between  two  chief  groups  of  muscles.  It  is  in  appearance  a 
strong,  white,  rounded,  glistening  cord.  Lay  it  bare  in  its  whole  length ; 
observe  that  it  gives  off  branches  to  the  muscles  of  the  back  of  the  leg, 

^  This  is  the  reverse  mode  of  distension  to  that  which  takes  place  during 
life,  when  the  lungs  are  distended  by  diminution  of  pressure  on  their  outer* 
surkceSi 


Appendix  of  Practical  Exercises,  277 

and  near  the  knee  divides  into  two  chief  branches,  which  go  to  supply 
the  parts  below  the  knee.  Dissect  the  nerve  upwards  towards  the  spinal 
cord,  and  observe  that  it  is  derived  from  three  or  four  nerves  which  unite 
to  form  what  is  known  as  the  sciatic  plexus}  Follow  these  nerves  to  the 
spinal  cord,  and  note  that  a  nerve  trunk  is  in  each  case  formed  by  the 
union  of  an  anterior  and  a  posterior  root  which  arise  separately  from 
the  side  of  the  spinal  cord.  Cut  a  piece  out  of  the  sciatic  nerve,  tease  it 
with  needles,  and  note  that  it  can  be  easily  split  up  longitudinally,  that  is, 
in  the  direction  in  which  the  fibres  run  in  it.  The  same  thing  may  next  be 
done  with  a  piece  of  muscle. 

Remove  the  skin  from  the  back  of  the  animal  and  from  the  back  of  the 
head  by  making  a  long  incision  in  the  middle  line,  and  turning  the  skin 
outwards  to  each  side.  Clear  the  muscles  away  from  the  vertebral  column 
in  the  back  and  neck,  and  then  with  a  pair  of  bone  forceps  snip  away  the 
laminae  of  the  vertebrae  so  as  to  expose  the  spinal  cord  throughout  its  length..^ 
Remove  also  the  upper  part  of  the  cranium  so  as  to  expose  the  brain,  and 
then  study  the  naked-eye  structure  of  the  cerebro-spinal  axis.  Note  the 
tough,  membrane  {dura  mater)  which  covers  the  brain  and  spinal  cord ;  re- 
move this  and  observe  the  convohitions  and  sulci  ou  the  surface  of  the  brain. 
Note  the  blood-vessels  which  are  carried  by  a  thinner  membrane  coating 
the  brain,  termed  the  pia  mater.  Remove  the  brain  and  cord,  commencing 
at  the  fore  part  of  the  brain.  The  cranial  nerves  must  be  cut  through 
in  order  to  do  this,  and  also  the  spinal  nerves.  Note  the  thick  optic 
nerves  which  meet  to  form  the  optic  chiasfjia.  Study  the  brain  and 
cord  with  the  aid  of  Chapter  XI.,  and  the  figures  given  there.  Note  the 
manner  in  which  the  nerve  roots  arise  from  the  brain  and  from  the  spinal 
cord.  Cut  a  slice  from  the  cerebral  cortex  and  observe  the  white  medullary 
centre  underlying  the  grey  cortex ;  slice  deeper  and  expose  the  cavity  or 
ventricle  within  the  cerebrum  ;  cut  into  the  basal  ganglia  seen  on  the  floor 
of  this  ventricle.  Slice  through  the  cerebellum  at  right  angles  to  the 
direction  of  the  convolutions,  and  observe  again  the  cortex  outside  and 
medulla  within,  giving  rise  to  the  appearance  known  as  the  arbor  vitce. 

Observe  also  the  pons  Varolii  connecting  the  two  cerebellar  hemispheres  ; 
the  crura  cerebri  conveying  the  fibres  in  two  great  masses  from  the  cerebrum 
towards  the  cerebellum  and  cord,  and  the  medulla  oblongata  or  spinal  bulb 
connecting  brain  and  cord.  Cut  through  the  spinal  cord  at  different  levels, 
and  note  the  grey  matter  in  the  centre  surrounded  by  the  white.  Observe 
also  the  increased  area  of  the  grey  matter  at  the  enlarged  parts  of  the  cord 
opposite  those  portions  where  the  nerves  to  the  limbs  are  given  off,  because 
here  more  nerve  cells  are  required  ;  also  the  comparatively  small  grey  area 
in  the  dorsal  region,  where  the  nerves  given  off  are  much  smaller.     Notice 

'  Bone  forceps  or  very  strong  scissors  will  be  required  in  doing  this  to 
snip  through  the  bones  which  conceal  these  nerve  roots. 

^  This  is  a  somewhat  difficult  dissection  for  a  beginner,  and  requires  the 
exercise  of  a  good  deal  of  patience. 


2/8  Elementary  Physiology. 

also  that  the  shape  of  the  entire  section  and  the  contour  of  the  grey  matter 
vary  in  the  different  regions,  thus  making  it  easy  to  distinguish  sections  of 
cord  from  cervical,  dorsal,  and  lumbar  regions  respectively. 

Finally,  clear  away  the  flesh  from  about  one  or  two  of  the  joints,  examine 
these,  dislocate  them  and  learn  their  construction. 

2.  Make  a  dissection  of  a  frog.  In  a  cold-blooded  animal  tissues  re- 
main alive  for  a  longer  time  after  the  death  of  the  animal  than  is  the  case 
with  mammals,  and  hence  many  important  physiological  facts  can  be  ascer- 
tained. A  small  depression  may  be  seen  behind  the  head,  almost  lying  at 
the  apex  of  an  imaginary  equilateral  triangle  with  the  line  joining  the  two 
eyes  as  base.  Insert  a  pin  at  this  depression  and  pith  the  brain  of  the 
animal  and  its  spinal  cord,  so  killing  it  by  destroying  the  nervous  system. 

Pin  the  animal  out  on  its  back  upon  a  cork  board,  and  remove  the  skin 
from  the  thorax  and  abdomen.  Next  remove  the  front  of  the  thorax,  taking 
care  not  to  go  too  deep,  so  as  to  avoid  injuring  the  heart  which  lies  under- 
neath. The  heart  is  then  exposed,  and  is  seen  beating  within  the  pericardium. 
Carefully  snip  away  the  pericardium,  and  observe  the  beating  of  the  heart  ; 
notice  that  the  auricles  beat  first  and  then  the  ventricle,  followed  by  a  pause.  ^ 
If  the  lung  should  happen  to  be  distended,  observe  that  it  is  of  a  much 
simpler  type  than  in  mammals,  being  simply  an  air  sac  on  the  wall  of  which 
blood-vessels  ramify.  Snip  into  the  lung  with  scissors,  and  it  at  once  collapses. 
Remove  the  skin  from  the  posterior  part  of  the  body  and  look  for  the  sciatic 
nerve  at  the  back  of  the  thigh.  Expose  the  nerve  for  some  distance,  then 
pinch  it  with  forceps,  and  note  that  the  muscles  at  the  back  of  the  leg 
beneath  the  knee  contract  vigorously  (mechanical  stimulation)  ;  cut  the 
nerve  through  with  scissors  below  the  point  previously  pinched,  and  note 
that  the  muscles  again  contract.  Dissect  out  the  corresponding  nerve  on 
the  other  side,  tie  it  tightly  near  the  upper  end  (the  muscles  contract) ;  now 
cut  it  above  the  point  at  which  it  has  been  tied — no  contraction  takes  place 
because  the  physiological  continuity  of  the  nerve  has  been  interrupted.  Cut 
the  nerve  below  the  point  at  which  the  ligature  was  tied,  apply  a  crystal  of 
salt  to  the  cut  end,  and  note  that  the  muscles  commence  contracting  and  go 
on  twitching,  because  the  nerve  is  stimulated  by  the  salt  (chemical  stimula- 
tion). Cut  the  nerve  beneath  the  place  at  which  the  salt  has  acted  upon  it,  and 
these  twitchings  cease.  Hold  a  zinc  and  copper  wire  in  contact  with  your 
fingers,  and  place  the  two  wires  across  the  nerve  at  a  short  distance  apart ; 
each  time  the  nerve  is  touched  by  the  wires,  the  muscles  which  it  supplies 
contract  (electrical  stimulation). 

3.  Observe  through  the  microscope  the  circulation  of  the  blood  in  the 
thin  web  of  the  frog's  foot,  or  in  the  tail  of  a  tadpole,  minnow,  or  other 
small  fish.  The  foot  should  be  tied  or  pinned  so  that  the  web  is  spread  out 
over  a  round  hole  in  a  piece  of  cork  board  or  wood,  and  then  the  prepara- 
tion should  be  brought  under  the  low  power  of  the  microscope.  In  the  case 
of  the  tadpole,  this  may  be  wrapped  round  with  wet  filter-paper  and  then 


'  There  is  only  one  ventricle  in  the  frog,  instead  of  two  as  in  mammals. 


Appendix  of  Practical  Exercises.  I'jc) 

placed  upon  a  microscope  slide  so  that  the  tail  lies  under  a  low-power 
objective.  Notice  that  the  red  blood  corpuscles  move  rapidly  down  the 
central  part  of  the  vessel  observed,  while  the  white  corpuscles  roll  sluggishly 
along  the  wall  of  the  vessel.  Bring  a  small  artery  into  the  field  of  view  and 
observe  the  pulsatile  flow,  the  beats  of  which  correspond  to  the  heart-beats. 

4.  Listen  to  the  heart-sounds  in  another  person  by  placing  your  ear 
opposite  the  proper  part  of  the  chest  wall.  At  the  same  time,  feel  the 
pulse  and  note  that  it  is  synchronous  with  the  beat  of  the  heart.  Count  the 
pulse-beats  per  minute;  ask  the  person  to  take  a  short  run,  then  listen  to 
the  heart,  and  feel  and  count  the  pulse  once  more.  Also  count  the  number 
of  respirations  per  minute,  before  and  after  a  run,  on  another  person  who 
is  not  aware  of  what  you  are  doing,  and  note  the  increase  in  rate. 

5.  Perform  the  experiment  upon  a  long  vein  of  the  arm  described  on 
p.  112. 

6.  Place  three  fingers  of  one  hand  upon  the  radial  artery  at  the  wrist ; 
the  pulsations  of  the  artery  can  be  distinctly  felt  by  all  three  fingers.  Com- 
press the  artery  strongly  with  the  middle  finger,  and  the  pulse  is  now  felt  by 
the  finger  towards  the  upper  arm,  but  not  by  the  finger  next  the  wrist,  thus 
showing  clearly  that  the  pulse  is  propagated  from  the  heart. 

7.  Obtain  an  uninjured  sheep's  heart  from  a  butcher,  and  make  the 
following  experiment  to  demonstrate  the  action  of  the  semilunar  valves. 
Take  two  glass  tubes,  about  half  an  inch  in  diameter  and  each  about  eighteen 
inches  long,^  and  tie  one  securely  into  the  aorta  and  the  other  into  one  of 
the  pulmonary  veins,  and  afterwards  tie  the  other  pulmonary  veins.  Or, 
one  tube  may  be  tied  into  the  pulmonary  artery  and  the  other  into  one  of 
the  vence  cavce,  the  other  vein  being  tied  up  as  before.  Now  pour  water 
into  the  tube  attached  to  the  vein,  both  tubes  being  held  vertically,  and 
alternately  compress  and  relax  the  ventricles  by  squeezing  with  the  hand. 
The  water  falls  in  the  tube  which  is  tied  in  the  vein,  and  rises  in  the  tube 
tied  in  the  artery,  being  upheld  during  the  intervals  of  relaxation  by  the 
closed  semilunar  valves.  This  illustrates  the  action  of  these  valves 
during  life. 

8.  Carry  out  with  the  same  heart  the  dissections  described  on  pp. 
1 15-120. 

9.  Make,  and  examine,  the  histological  preparations  of  blood  corpuscles 
described  on  p.  121. 

10.  Instruct  a  butcher  to  draw  off  a  quantity  of  blood  into  a  vessel  when 
killing  an  animal,  and  then  later  study  the  character  of  the  contents  of  this 
vessel.  The  clot  is  red  outside,  but  black  when  cut  into  ;  also,  when  the 
cut  surface  is  exposed  for  some  time  to  the  air  it  turns  red  (see  p.  187). 

11.  Obtain  also  some  whipped  blood,  and  perform  the  experiments 
indicated  on  p.  187. 

'  The  tubes  should  be  slightly  pulled  out  in  a  blow-pipe  flame  close  to 
one  end,  so  as  to  form  a  shoulder  round  which  the  ligature  can  be  firmly 
applied. 


28o  ■     Elementary  Physiology. 

12.  Pour  off  some  of  the  serum  which  has  exuded  from  the  clot  obtained 
in  experiment  lo,  and  dilute  it  with  about  three  times  its  volume  of  water. 
With  the  fluid  so  obtained  perform  the  following  tests,  which  are  characteristic 
tests  for  prpteids  : — 

{a)  Add.  a  trace  of  acetic  acid,  and  heat ;  before  boiling  commences  a 
white  coagulum  is  thrown  down.  This  is  what  happens  to  the  white  of  egg 
when  an  &g^  is  boiled,  and  is  termed  heat  coagulation. 

{b)  Add  a  single  drop  of  a  dilute  solution  of  copper  sulphate,  and  after- 
wards excess  of  caustic  potash,  and  the  solution  will  turn  a  violet  colour 
(Biuret  test). 

{c)  Add  strong  nitric  acid  ;  the  solution  turns  yellow  and  gives  a  white 
precipitate  which  turns  yellow  on  boiling.  Allow  the  solution  to  cool,  and 
then  add  excess  of  ammonia,  when  the  solution  will  turn  orange-coloured 
(xantho-proteic  test). 

{d)  Add  excess  of  alcohol  and  a  white  precipitate  will  be  thrown  down. 

13.  If  possible,  carry  out  the  separations  described  on  p.  128. 

14.  Make  some  starch  paste  by  powdering  a  small  quantity  of  starch  ^ 
and  then  boiling  with  water.  If  a  drop  of  this  paste  be  added  to  a  drop  of 
a  dilute  solution  of  iodine,  a  deep  blue  colour  is  the  result.  Now  add  some 
saliva  from  the  mouth  to  a  quantity  of  this  starch  paste,  keep  the  mixture 
warm,  but  not  too  hot,^  and  test,  by  means  of  the  iodine  solution,  drops 
taken  from  it  from  time  to  time.  It  will  be  found  that  the  blue  coloration 
after  a  time  no  longer  appears,  and  finally,  after  a  transitory  stage  in  which 
a  red  is  obtained,  no  coloration  whatever  is  produced  by  the  iodine.  This 
experiment  shows  that  the  starch  has  been  converted  into  something  else  by 
the  action  of  the  saliva,  and  if  some  of  the  solution  be  now  taken  and  tested 
by  adding  a  few  drops  of  copper  sulphate  and  excess  of  caustic  potash  it 
will  be  found  that  the  copper  salt  is  reduced.  In  fact,  as  can  be  shown  by 
more  elaborate  experiments,  a  reducing  sugar  called  maltose,  mixed  with 
certain  bodies  intermediate  in  chemical  nature  between  starches  and  sugars, 
and  called  dextiins,  has  been  produced  by  the  action  of  the  saliva  on  the 
starch  (see  p.  146). 

15.  Feed  an  animal  on  food  containing  fat,  such  as  fat  meat,  and  kill  it 
after  an  interval  of  about  five  hours.  Open  the  abdomen  and  observe  the 
milky  lacteals  in  the  mesentery.  Open  the  thorax  and  search  for  the  thoracic 
duct,  which  is  charged  with  milky  fluid  and  can  on  this  account  easily  be 
found  ;  trace  this  vessel  to  its  entrance  into  the  junction  of  the  subclavian 
and  jugular  veins  in  the  neck.  Note  how  richly  it  is  supplied  with  valves, 
which  are  shown  by  the  swellings  upon  it. 

16.  Feed  a  rabbit  freely  on  rice  or  carrots  for  one  or  two  days,  and  then 
kill  it  four  or  five  hours  after  a  meal.  Rapidly  cut  out  the  liver  and  throw 
it  in  small  pieces  into  a  vessel  containing  boiling  water  just  acidulated  with 


^  Half  a  teaspoonful  will  make  a  cupful  of  paste. 

-  The  mixture  should  be  kept  at  such  a  temperature  that  the  finger  can 
be  kept  in  it  without  discomfort,  that  is  to  say,  at  about  body-temperature. 


Appendix  of  Practical  Exercises.  281 

acetic  acid.  This  process  coagulates  the  proteid  present  in  the  liver.  Take 
the  pieces  of  coagulated  liver  out  of  the  boiling  water  after  they  have  been 
immersed  for  about  a  minute,  and  grind  them  up  with  some  cold  water  in  a 
mortar.  Filter  through  muslin,  and  a  very  opalescent,  almost  milky  fluid 
will  be  obtained  which  is  rich  in  glycogen  or  animal  starch.  If  the  liver  be 
left  for  some  time  after  death  before  extracting  as  described  above,  all  the 
glycogen  becomes  converted  into  grape  sugar.  A  similar  change  can  be 
induced  in  the  glycogen  solution  by  heating  it  with  acids  or  with  saliva  or 
pancreatic  extract.  If  dilute  iodine  solution  be  added  to  the  glycogen  solu- 
tion a  deep  brown  or  port-wine  colour  is  produced,  which  disappears  on 
heating  and  reappears  on  cooling.  The  glycogen  may  be  precipitated  from 
solution  by  the  addition  of  60  per  cent,  of  alcohol,  and  so  obtained  as  a 
white  amorphous  powder. 

This  experiment  shows  that  the  liver  stores  up  excess  of  carbohydrate 
in  the  form  of  a  variety  of  starch. 

17.  Obtain  some  ox-bile  from  a  butcher  and  perform  the  chemical  tests 
for  bile  salts  and  bile  pigments  described  on  pp.  163  and  164. 

18.  Fit  a  bottle  with  a  cork  through  which  two  glass  tubes,  bent  once 
at  right  angles,  pass,  one  tube  reaching  to  the  bottom  of  the  bottle,  and 
the  other  only  just  passing  through  the  cork.  Fill  the  bottle  nearly  full  of 
lime-water,  and  place  the  cork  in  position,  apply  your  mouth  to  the  shorter 
tube,  and  suck  so  that  atmospheric  air  bubbles  through  the  lime-water  ;  no 
change  takes  place  in  the  lime-water  unless  the  process  be  continued  for  a 
very  long  time.  Now  apply  your  mouth  to  the  longer  tube,  and  blow  air 
from  your  lungs  through  the  lime-water.  One  or  two  breaths  will  prove 
sufficient  to  give  a  white  precipitate  of  calcium  carbonate  in  the  lime-water  ; 
this  is  produced  by  the  action  of  the  carbon  dioxide  liberated  in  the  lungs. 

19.  Breathe  against  a  clear  cold  mirror  ;  it  becomes  clouded  by  the 
moisture  deposited  from  the  air  coming  from  the  lungs,  which  is  saturated 
M'ith  water  vapour  at  the  temperature  of  the  body,  and  hence  causes  a 
deposit  of  water  upon  any  cold  surface  with  which  it  comes  in  contact. 

20.  Tie  a  glass  tube,  connected  by  indiarubber  tubing  to  a  U-shaped 
tube  containing  a  coloured  fluid,  into  the  trachea  of  a  dead  animal.  Now 
cut  into  the  thorax  and  thus  allow  the  full  atmospheric  pressure  to  act  upon 
the  lungs  ;  these  partially  collapse,  and  the  coloured  fluid  is  driven  up  in 
the  distal  limb  of  the  U-tube.  This  shows  that  the  lungs  are  held  distended 
by  a  partial  vacuum  existing  within  the  thorax,  and  at  the  same  time  points 
to  how,  when  the  volume  of  the  thorax  is  enlarged  during  life  that  of  the 
lungs  must  also  be  enlarged  on  account  of  the  pressure  of  the  atmospheric 
air  from  without  acting  down  the  trachea,  bronchi,  and  bronchioles. 

21.  Perform  the   experiment  on  reflex  activity    indicated  on  p.    239.^ 

^  The  frog's  head  should  be  removed  with  scissors,  leaving  the  upper 
end  of  the  spinal  cord  exposed,  and  the  remainder  of  the  animal  should  be 
vertically  suspended  by  means  of  a  bent  pin  used  as  a  hook  from  some  con- 
venient support. 


282  Elementary  Physiology. 

Two  per  cent,  solution  of  acetic  acid  may  be  employed  for  the  stimula- 
tion. 

By  diluting  the  acid,  find  a  strength  such  that  the  leg  is  not  drawn  up 
out  of  a  beaker  containing  the  acid  until  20  to  30  seconds  have  elapsed. 
Wash  the  leg  free  again  from  acid  by  dipping  it  in  water.  Now  apply  a 
pinch  of  common  salt  to  the  cut  end  of  the  spinal  cord,  and  after  a  pause 
of  a  few  seconds,  again  test  the  time  that  elapses  before  the  leg  is  with- 
drawn from  the  same  acid.  It  will  usually  be  found  that  the  interval  is 
enormously  increased.  This  experiment  illustrates  inhibition  ;  that  is,  it 
shows  that  reflexes  can  be  stopped  or  delayed  by  controlling  impulses 
coming  from  the  higher  centres.  For  the  salt  stimulates  nerve  fibres  in  the 
cord  which  in  a  normal  condition  of  the  animal  would  be  connected  with 
nerve  cells  in  the  brain,  and  originates  impulses  down  these  fibres  which 
for  a  time  stop  the  reflex  act  due  to  the  irritation  of  the  lower  cells  by  the 
acid. 

22.  Test  with  a  blunt-pointed  pair  of  compasses,  the  various  distances 
apart  at  which  the  two  points  can  be  appreciated  as  distinct,  over  different 
regions  of  the  skin  (see  p.  250). 

23.  Obtain  two  or  three  ox's  or  pig's  eyes  from  a  butcher  and  make 
dissections  of  them. 

Clear  away  from  the  eyeball  all  adhering  fat  and  muscles,  and  note  the 
place  where  the  optic  nerve  enters,  and  the  external  appearance  of  the  eye- 
ball. Insert  one  blade  of  a  pair  of  scissors  through  the  sclerotic  coat 
about  a  quarter  of  an  inch  behind  the  corneo-sclerotic  junction,  and  cut 
completely  round  parallel  to  this  junction,  and  about  a  quarter  of  an  inch 
behind  it,^  thus  dividing  the  eyeball  into  an  anterior  and  a  posterior  part. 
When  the  two  parts  are  pulled  apart  the  vitreous  humour  is  exposed,  and 
is  seen  to  be  a  beautifully  clear  jelly-like  mass.  The  vitreous  humour 
usually  adheres  to  the  front  portion,  but  can  easily  be  detached  and 
examined.  After  the  vitreous  humour  has  been  removed  examine  the  front 
portion  from  the  inner  side.-  Observe  the  crystalline  lens  lying  in  the 
middle  opposite  to  the  pupil  and  its  mode  of  attachment  by  the  ciliary 
processes  all  round  the  margin  to  the  anterior  portion  of  the  choroid  coat 
which  is  seen  surrounding  it  and  forming  a  black  internal  coating  to  the 
eyeball.^  The  lens  is  enclosed  in  a  capsule,  which  is  very  delicate  and 
easily  ruptured.  Remove  the  lens  by  pressing  against  it  with  the  handle  of 
a  scalpel  at  one  side,  or  by  passing  a  pin  round  the  margin.  Hold  the  lens 
up  to  the  light  and  notice  how  clear  and  transparent  it  is ;  but  on  looking 
through  it,  especially  some  time  after  the  death  of  the  animal,  three  radial 
lines  may  be  seen  which  meet  at  angles  of  120°  at  the  centre.     These  three 

^  It  is  advantageous  to  do  this  in  a  vessel  of  water,  the  parts  are  then 
better  seen  and  less  injured  in  the  dissection. 

"  It  may  be  removed  from  the  water  for  this  purpose. 

^  Identify  these  various  structures  by  the  aid  of  the  diagram  given  on 
p.  267. 


Appendix  of  Practical  Exercises.  283 

lines  are  due  to  the  structural  arrangements  of  the  lens.  By  breaking  up, 
in  a  small  quantity  of  water,  with  needles  it  may  be  seen  that  the  lens  is 
made  up  of  an  immense  number  of  transparent  fibres  running  from  front  to 
back.^  These  fibres  are  arranged  in  three  equal  bundles,  and  the  tri-radiate 
arrangement  described  above  is  due  to  the  junctions  of  these  bundles. 
Examine  the  anterior  portion  again  after  the  removal  of  the  lens.  Observe 
the  ii'is,  with  its  central  aperture,  \h(t  picpil,  for  the  admission  of  the  light. 
The  pupil  is  lenticular  in  shape  in  the  ox's  or  pig's  eye,  and  not  circular  as 
in  the  human  eye.  The  back  surface  of  the  iris  is  always  black,  from  the 
continuation  of  the  choroid  containing  black  pigment  over  it,  the  front 
surface  is  variously  coloured  in  different  individuals.  Note  how  the  outer 
margin  of  the  iris  is  attached  all  round  at  the  junction  of  the  cornea  and 
sclerotic  coat.  Rub  away  with  a  blunt  knife  the  black  pigment  of  the 
choroid  lying  behind  the  junction  of  the  cornea  with  the  sclerotic,  and  you 
will  expose  a  circular  ridge,  lying  all  round  the  junction.  This  ridge  is 
formed  by  the  fibres  of  the  ciliary  muscle.  Make  a  cut  forwards  at  one 
point  with  a  sharp  knife,  so  as  to  pass  at  right  angles  through  the  corneo- 
sclerotic  junction.  Some  of  the  fibres  of  the  ciliary  muscle  run  circularly 
parallel  to  the  junction  of  cornea  and  sclerotic  (circular  fibres),  another  set 
run  at  right  angles  to  the  junction,  and  are  attached  in  front  at  the  junction 
and  behind  to  the  choroid  coat.  It  is  evident  that  when  the  fibres  of  the 
ciliary  muscle  contract,  the  choroid  will  be  pulled  forward,  and  hence  the 
processes  (ciliary  processes),  which  were  previously  observed  attacking 
the  lens  all  round  to  the  choroid,  will  be  slackened.  The  lens  will,  in 
consequence,  not  be  so  much  flattened  against  the  vitreous  humour  by  the 
pull  of  these  processes,  and  will  become  more  rounded.  This  happens 
when  we  look  at  a  near  object. 

Examine  next  the  posterior  half.  Observe  the  retina  lying  innermost  of 
the  three  coats  ;  ^  this  coat  is  usually  detached  from  the  choroid  except 
where  the  optic  nerve  enters,  and  is  seen  floated  up  by  the  water  in  which 
it  is  immersed  as  a  yellow-coloured  almost  transparent  film.  Outside  this 
is  seen  the  black  choroid  coat,  and  outermost  of  all  there  is  the  sclerotic 
coat, 

24.  If  an  eye  can  be  obtained  in  a  fresh  enough  condition,'  the  follow- 
ing experiment  may  be  made.  Cut  a  circular  opening  out  of  the  back  part 
of  the  eye,  including  the  optic  nerve,  and  about  one-third  of  the  posterior 
surface  of  the  eyeball.  Take  care  that  the  opening  is  not  large  enough  to 
allow  the  vitreous  humour  to  escape.     Now  apply  a  piece  of  greased  paper 


^  These  fibres  can  be  better  seen  in  the  case  of  a  lens  which  has  been 
hardened  for  some  days  in  two  per  cent,  solution  of  potassium  chromate. 

-  In  the  eye  of  the  ox  there  is  a  thin  glistening  coat,  of  a  green  colour, 
\\hich  is  termed  the  tapehim,  to  the  inside  of  the  retina.  The  purpose  of 
this  coat  is  unknown.     It  is  not  present  in  the  human  eye. 

'  The  cornea  rapidly  loses  its  transparency  after  death,  and  hence  the 
above  experiment  is  only  successful  with  a  fresh  eye. 


284  Elementary  Physiology. 

against  the  opening  and  turn  the  eyeball  so  that  light  from  a  window  enters 
it  in  front.  An  image  of  the  window  will  be  formed  upon  the  paper 
behind. 

25.  The  shape  of  the  blind  spot  may  be  roughly  mapped  out  by  the 
following  method.  Make  an  ink  spot  upon  a  piece  of  white  paper,  close 
the  left  eye,  and  place  the  right  eye  vertically  opposite  the  ink  spot  and 
about  ten  inches  distant  from  it.  Move  the  point  of  a  pencil  or  pen  to  the 
right,  away  from  the  ink  spot,  keeping  the  eye  always  direct  upon  the  ink 
spot,  and  not  upon  the  pen  point,  until  the  point  just  ceases  to  be  visible, 
and  mark  this  spot ;  continue  moving  the  pen  point  still  further  to  the  right 
until  it  again  becomes  visible,  and  mark  this  point  also.  This  gives  the 
horizontal  diameter  of  the  blind  spot,  and  the  colour  may  be  marked  in  by 
moving  the  pen  point  up  and  down  above  and  below  this  line  at  inter- 
mediate points,  and  marking  in  each  case  the  spot  at  which  it  is  just  visible. 

26.  Try  Mayer's  experiment,  described  on  p.  274. 


TEST   QUESTIONS^ 

1.  Describe  the  life-history  of  the  simplest  animal  organism. 

2.  State  the  similarity  which  exists  between  the  simplest  type  of  living 
animal  and  the  more  complex  types. 

3.  Define  the  terms  "tissue,"  "organ,"  and  "gland." 

4.  Describe  a  typical  vertebra,  and  state  in  general  terms  how  this 
structure  is  modified  in  various  parts  of  the  vertebral  column. 

5.  Describe  the  articulation  of  the  lower  jaw  with  the  skull,  and  the 
movements  which  take  place  at  this  joint. 

6.  Describe  the  bony  framework  of  the  thorax,  and  the  manner  in  which 
the  volume  of  the  thorax  is  altered  during  respiration. 

7.  State  the  bones  and  joints  which  correspond  to  one  another  in  the 
upper  and  lower  limbs. 

8.  Give  a  classification  of  joints,  stating  the  nature  of  the  movement  in 
each,  and  illustrating  by  examples. 

9.  Name  the  different  classes  of  muscular  tissue,  state  where  each  kind 
is  to  be  found,  its  microscopic  appearance,  and  in  general  terms  its  use  in 
the  body. 

10.  How  is  the  erect  position  of  the  body  maintained  ^ 

11.  Enumerate  the  viscera  found  in  the  thorax  and  in  the  abdomen 
respectively,  and  state  their  position  in  these  cavities. 

12.  Describe  the  course  of  the  aorta,  and  name  the  principal  branches 
which  it  gives  off. 

13.  Beginning  at  the  aorta,  describe  the  path  of  a  blood  corpuscle  which 
makes  a  complete  circuit.  What  is  the  longest,  and  what  the  shortest,  path 
which  such  a  corpuscle  could  take  ? 

14.  State  in  general  terms  the  purposes  served  in  the  body  by  the 
circulation  of  the  blood. 

15.  Describe  the  walls  of  the  blood-vessels  in  the  cases  of  arteries, 
capillaries,  and  veins,  pointing  out  similarities  and  differences. 

16.  "What  is  meant  by  the  "  heart  sounds  "  ?  and  how  are  these  caused  ? 

1 7.  Why  are  the  auricles  thin-walled  and  the  ventricles  thick-walled  ?  and 
why  is  the  wall  of  the  left  ventricle  thicker  than  that  of  the  right  ventricle  ? 

18.  Where  in  the  circuit  does  the  greatest  fall  in  blood-pressure  take 
place  ?  and  what  is  the  cause  of  this  rapid  fall  ? 

^  These  questions  are  intended  to  point  out  important  points,  which 
should  be  specially  studied  and  understood  ;  they  should  not  be  attempted 
until  the  book  has  been  once  carefuUv  read  over. 


286  Element a7'y  Physiology. 

19.  What  is  the  cause  and  the  nature  of  the  pulse  in  the  arteries  ?  and 
why  does  the  blood-flow  become  uniform  in  the  veins  ? 

20.  What  determines  the  relative  velocity  of  the  blood-flow  in  arteries, 
capillaries,  and  veins  respectively,  and  what  the  local  velocity  in  any 
particular  area  ? 

21.  Describe  the  venous  valves,  and  state  what  use  they  serve.  Which 
veins  have  no  valves  ? 

22.  How  would  you  identify  the  different  chambers  in  an  excised  heart  ? 
What  cuts  would  you  make  in  it  in  order  to  expose  the  interior  of  each 
chamber  ?     What  differences  are  found  in  the  two  sides  ? 

23.  Describe  the  tri-cuspid  valve.  What  are  the  uses  of  the  chorda 
tendijice  and  musculi  papillares  ? 

24.  Describe  the  corpuscles  found  in  the  blood,  and  state  their  functions. 

25.  What  are  the  names  given  to  the  fluid  part  of  the  blood  before  and 
after  clotting  respectively  ?  What  substances  are  dissolved  in  this  fluid  part 
of  the  blood  ? 

26.  Enumerate  the  conditions  which  respectively  hasten  and  retard 
coagulation  of  the  blood. 

27.  State  the  part  which  plants  play  in  ^^reparing  the  food  of  animals. 

28.  Into  what  classes  or  groups  can  the  substances  present  in  food  be 
divided,  and  what  are  the  characteristics  of  each  class  ?     Give  examples. 

29.  Give  the  position  of  the  salivary  glands.  What  is  the  action  of 
saliva  on  food  ? 

30.  What  is  meant  by  the  term  ^^  enzyme'''' 1  Enumerate  the  digestive 
enzymes,  stating  in  which  digestive  secretion  each  is  found,  and  discuss 
briefly  their  action  on  food. 

3 1 .  State  the  general  characteristics  of  enzymic  action. 

32.  Describe  the  two  types  of  gastric  gland,  and  state  what  constituents 
of  the  gastric  juice  are  probably  secreted  by  each  type, 

33.  Enumerate  and  briefly  describe  the  coats  of  the  wall  of  the  small 
intestine. 

34.  Describe  a  vilhis  of  the  small  intestine.  What  use  is  served  by  the 
villi  ? 

35.  Describe,  first,  the  naked-eye  appearance  of  the  liver  ;  secondly, 
its  minute  structure  ;  and  thirdly,  the  nature  and  arrangement  of  its  blood- 
supply. 

36.  Describe  the  physical  properties  of  bile ;  enumerate  its  chief 
chemical  constituents,  and  state  veiy  briefly  the  properties  of  each  of  these. 

37.  How  have  the  bile  pigments  been  shown  to  be  related  to  haemoglo- 
bin? What  information  does  this  give  as  to  their  source  in  the  body? 
What  is  their  ultimate  fate  ? 

38.  What  uses  do  the  bile  salts  serve  in  the  body  ?  W^hat  is  meant  by 
the  term  ^''circulation  of  the  bile  "  ? 

39.  What  is  meant  by  the  "glycogenic  function"  of  the  liver?  State 
the  evidence  that  the  liver  can  also  act  as  a  temporary  storehouse  for  other 
food-stuffs  than  carbohydrates. 


Test  Questions.  287 

40.  What  are  the  meanings  of  the  terms  "metabolism,"  "katabolism," 
and  "anabolism  "  ? 

41.  Where  is  fat  chiefly  stored  in  the  body  ?  Describe  the  modifications 
which  the  cells  undergo  as  they  become  charged  with  fat. 

42.  To  what  extent  can  the  various  kinds  of  food-stuff  replace  one 
another,  and  be  converted  into  one  another  in  the  metabolic  processes  ? 

43.  Describe  concisely  the  structure  of  the  respiratory  apparatus. 

44.  Describe  the  changes  which  [a)  the  air  and  (/;)  the  blood  undergo 
in  the  lungs  in  the  process  of  respiration.  How  may  some  of  these  changes 
be  demonstrated? 

45.  In  what  manner  is  the  oxygen  held  in  the  blood?  What  circum- 
stances determine  the  amount  of  oxygen  so  held  ? 

46.  Why  does  the  blood  lose  oxygen  and  take  up  carbon  dioxide  in 
passing  through  the  capillaries  of  the  tissues  ?  Describe  the  manner  of  the 
exchange,  as  far  as  is  known,  between  the  tissue  cells  and  the  blood. 

47.  What  is  asphyxia  I  Enumerate  some  of  the  ways  in  which  it  may 
be  caused. 

48.  How  is  the  temperature  of  the  body  maintained  constant  ? 

49.  State  the  channels  by  which  the  waste  of  the  body  is  removed,  and 
mention  the  chief  waste  products  removed  by  each. 

50.  Describe  the  minute  structure  of  the  skin.  Briefly  describe  the 
structure  of  a  nail,  of  a  hair  follicle,  and  of  a  sweat  gland. 

51.  Name  the  parts  of  the  urinary  system,  and  state  their  relationship  to 
one  another. 

52.  Describe  the  naked-eye  appearance  of  a  kidney  as  seen  in  longitu. 
dinal  section. 

53.  Describe  the  course  of  a  uriniferous  tubule,  and  the  structure  of  the 
cells  lining  it  at  various  parts. 

54.  Describe  the  arrangement  of  the  blood-supply  of  the  kidney. 

55.  State  what  you  know  as  to  the  manner  in  which  the  urine  is  secreted 
in  the  kidney. 

56.  What  evidence  have  we  that  urea  is  formed  in  the  liver  and  not  in 
the  kidney  ?  From  what  substances  is  urea  probably  formed  in  the  liver  ? 
(See  p.  165.) 

57.  What  is  the  chemical  formula  of  urea,  and  what  is  its  nature  as  a 
chemical  compound  ?  What  gives  this  substance  its  chief  importance  to  the 
physiologist  ? 

58.  Enumerate  the  other  important  constituents  of  normal  urine. 

59.  Under  what  conditions  do  abnormal  constituents  appear  in  the 
urine,  and  what  does  their  presence  under  such  conditions  teach  us  as  to 
the  function  of  the  kidneys  ? 

60.  State  in  general  terms  the  work  performed  in  the  body  by  the 
nervous  system. 

61.  Enumerate  the  parts  of  the  central  nervous  system,  and  very  briefly 
describe  each  part. 

62.  Why  is  it  that  when  one  side  of  the  brain  is  injured,  effects  are 
produced  upon  the  opposite  side  cf  the  body  ? 


288  Elementary  Physiology. 

63.  Enumerate  the  cranial  nerves,  and  very  briefly  state  the  use  of 
each  pair. 

64.  Describe  the  sympathetic  nervous  system,  and  state  in  general  terms 
w^hat  is  the  nature  of  its  action  in  the  body. 

65.  Describe  a  medullated  nerve  fibre. 

66.  Describe  a  simple  reflex  act. 

67.  What  is  meant  by  the  term  "  co-ordination  "  ? 

68.  Describe  the  condition  of  an  animal,  such  as  a  frog,  from  which  the 
cerebral  hemispheres  have  been  removed. 

69.  State  concisely  the  functions  of  the  different  parts  of  the  brain  and  cord. 

70.  What  is  the  chief  difference  between  the  so-called  common  and 
special  sensations  ? 

71.  What  is  meant  by  the  "  muscular  sense  "  ?  By  what  nerve  endings 
is  it  probably  called  into  action  ? 

72.  What  is  the  law  of  "  specific  sensation  "  ? 

73.  What  is  the  Weber-Fechner  law  ? 

74.  State  the  various  injuries  to  the  visual  apparatus  by  which  blindness 
may  be  occasioned. 

75.  What  evidence  is  there  that  there  are  nerve  endings  in  the  skin 
which  are  connected  with  different  kinds  of  sensation  ? 

76.  How  is  the  delicacy  of  touch  estimated  in  different  areas  of  the  skin  ? 
77-  Describe  the  histological  appearance  of  the  end  organs  of  taste  and 

of  smell. 

78.  Name  the  four  taste  sensations  which  are  usually  referred  to  as 
primitive.  How  are  these  modified  so  as  to  give  rise  to  the  many  flavours 
which  we  experience  ? 

79.  Describe  the  structure  of  the  middle  ear,  and  the  movements  of  the 
auditory  ossicles. 

80.  Describe  the  structure  of  the  cochlea. 

81.  Enumerate  the  changes  which  take  place  between  the  arrival  of  a 
sound  wave  at  the  membrana  tympani  and  the  stimulation  of  the  nerve 
endings  in  the  organ  of  Corti. 

82.  From  what  defects  in  the  auditory  apparatus  may  deafness  arise? 

83.  Describe  the  semicircular  canals,  and  state  what  is  their  function. 

84.  Draw  a  diagram  of  a  section,  from  front  to  back,  of  the  eyeball 
and  mark  in  the  names  of  the  various  parts. 

85.  What  is  the  iris?  Why  does  the  pupil  vary  in  size,  and  how  are 
the  changes  in  size  produced  ? 

86.  What  changes  take  place  in  the  eye  when  the  attention  is  directed 
from  a  distant  to  a  near  object,  and  how  are  these  changes  brought  about  ? 

87.  Which  layer  of  the  retina  is  sensitive  to  light,  and  how  has  this  been 
shown  ? 

88.  What  is  the  "blind  spot,"  and  how  is  its  existence  demonstrated? 

89.  What  is  an  "  after-image  "  ? 

90.  What  is  meant  by  sirmdtaneous^  and  what  by  successive  contrast  ? 
Describe  an  experiment  to  show  the  effect  of  simultaneous  contrast* 


INDEX 


Abdomex,  32 

Absorption,  5,  151,  156,  i57 

of  carbohydrates,  156 

of  fatty  acids,  156 

ofproteids,  156 

Accommodation,  268,  269 
Acetabulum,  35 
Acidity  of  urine,  215,  216 
Acromion  process,  38 
Adipose  tissue,  168 
Afferent  nerves,  226 
After-image,  273 
Air,  alveolar,  176 

,  changes  in  respiration,  183 

,  complemental,  257 

,  composition  of,  182 

,  residual,  180 

,  stationary,  181 

,  supplemental,  181 

,  tidal,  181 

Alimentary  canal,  152-154 

Amoeba,  structure  of,  2 

Amoeboid  movements,  2,  3 

Amylolytic  ferment,  145 

Amylopsin,  149 

Anabolism,  definition,  9 

Animal  heat,  192-194 

Antero-lateral  tract,  236 

Anus,  57,  92 

Aorta,  82 

Aphasia,  241 

Apnoea,  189 

Aqueous  humour,  266 

Arterial  blood-pressure,  105 

Artery,  structure  of,  98,  99 

Articular  surface,  13 

Articulations  of  ankle,  50 

,  astragalus,  50 

of  atlas  and  axis,  21 


Articulations,  carpal,  43 

,  carpo-metacarpal,  43 

,  classification  of,  52-55 

of  elbow,  40 

of  face,  28 

of  foot,  50,  51 

of  hand,  44 

of  hip,  36 

of  knee,  48 

of  lower  jaw,  30 

of  pelvis,  35 

of  ribs,  32 

of  shoulder,  39 

of  skull  bones,  25 

of  thorax,  32 

of  vertebral  column,  18 

Asphyxia,  85,  188 

Assimilation,  5,  157 

Astragalus,  50 

Atlas,  21 

Auditory  meatus,  255 

ossicles,  movement  of,  261 

Auricle  of  heart,  81,  100,  117 

Auricular  appendages  of  heart,  114 
Automatic  action,  73 

Axis,  22 

Basilar  membrane,  259 
Bicuspid  tooth,  31 
Bile,  149,  162 

circulation,  163 

duct,  159,  162 

■  pigments,  163,  164 

salts,  162,  163 

,  tests  for,  163,  164 

BiUrubin,  163 
Biliverdin,  164 
Bladder,  urinary,  94 
Bhnd  spot,  270 

U 


290 


Index. 


Blood,  changes  in,  102,  184 

,  circulation  of  the,  81,  99-103 

,  coagulation  of,  124-127 

;  composition  of,  128,  129 

gases,  187 

serum,  128 

Blood- corpuscles,  red,  121-123 

,  white,  2,  122 

Blood-plasma,  124,  185 
Blood-pressure,  105,  106 
Blood-serum,  128,  129 
Bone,  cancellous,  14 

,  compact,  14 

,  structure  of,  13 

Bronchi,  79,  175 

,  structure  of,  177,  178 

C^CUM,  91 

Calcaneum,  51 

Calcium,  effect  on  clotting,  126 

Canine  tooth,  31 

Capillary,  structure  of,  65,  95 

Capitellum,  40 

Carbohydrates,  132,  133 

,  absorption  of,  156 

■,  digestion  of,  145 

Carbon   dioxide,    excretion   of,    185, 

186,  199 
Carotid  artery,  82 
Cartilage,  structure  of,  14,  15 
Cauda  equina,  227 
Caudate  lobe  of  liver,  89 
Cell,  the,  4 
Centre,  respiratory,  242 

,  speech,  242 

,  vasa  motor,  243 

Centres  in  spinal  cord,  244 
Cerebellar  peduncles,  222 
Cerebellum,  223 
Cerebral  convolutions,  220 

cortex,  221 

hemispheres,  219 

■  localization,  241 

Cerebrum,  function  of,  239-241 

,  removal  of,  239 

Cholalic  acid,  162 
Cholestearin,  163 
Chordae  tendinae,  116,  120 
Choroid  coat,  264 
Chyle,  148 


Ciliary  muscle,  264 

process,  268 

Circulation  of  the  blood,  81,  99 
Circumvallate  papillae,  252 
Clavicle,  38 

Coagulation  of  blood,  124 
Coccyx,  24 
Cochlea,  259 

Cold-blooded  animals,  193 
Colon,  90,  92 
Colourblindness,  274 

vision,  272 

Complemental  air,  181 
Complementary  colour,  273 
Conjunctiva,  203 
Contraction  of  heart,  102 

of  muscle,  70 

Contrast  phenomena,  274 
Co-ordination  of  movement  238,  273 
Cornea,  265,  268 
Corona  radiata,  222 
Corpora  Aurantii,  120 
Corpus  collosum,  219 
Corpuscles,  red,  121 

,  tactile,  235,  236,  249 

white,  2,  122 

Corti,  organ  of,  255,  259,  261 

,  tunnel  of,  261 

Cranial  cavity,  27 

nerves,  227-231 

Cranium,  25 
Creatinine,  216 
Crystalline  lens,  266 
Cuneform  bones,  51 
Cutaneous  sensation,  251 
Cutis  vera,  2co,  203 

Deafness,  263 
Decussation  of  pyramids,  224 
Defaecation,  57 
Degeneration  of  nerves,  235 
Deglutition,  137 
Dendrons,  235 
Dextrin,  146 
Diabetes,  168 
Diaphragm,  32 

,  action  of,  34 

Diarthrosis,  52 
Dicrotic  wave,  109 
Diet,  135-137 


Index. 


291 


Digestion,  145-149 
Duct,  bile,  159,  162 
Duct  of  Rivinus,  143 

of  Stensen,  142 

,  thoracic,  65,  79 

Ductless  glands,  7,  8 
Duodenum,  90 

Ear,  lobule  of,  255 

Efferent  nerve  fibres,  227 

Elbow,  articulation  of,  40 

Electrodes,  70 

Endolymph,  258 

Endplate  of  nerve,  6j,  235 

Enzymes,  144-149 

Epidermis,  200 

Erect  position,  mechanism  of,  j'j 

Eustachian  tube,  257 

Excretion  of  carbon  dioxide,  199 

Expiration,  34 

Eye,  anterior  chamber,  266 

,  movements  of,  264 

,  muscles  of,  264,  265 

,  posterior  chamber,  266 

Face,  bones  of,  28 

Fascia,  56 

Fats,  134,  168-171 

,  absorption  of,  156 

,  digestion  of,  149 

Femur,  37,  45,  47 
Fenestra  ovalis,  257 

rotundis,  257 

Ferments,  144-149 

Fibrin  ferment,  127,  145,  148 

Fibrinogen,  125,  127 

Fibula,  50 

Filum  terminale,  227 

Fissure  of  Rolando,  241 

Foramen  magnum,  28 

Fossa  ovalis,  114 

Fovea  centralis,  271 

Gall  bladder,  89 
Gastric  juice,  146,  148 
Glands,  6 

,  ductless,  7,  8 

,  gastric,  146,  147 

,  lachrymal,  6,  7,  265 


Glands  of  Lieberkiihn,  139,  150 

,  mucous,  139 

,  parotid,  142 

,  racemose,  140 

,  salivary,  7,  140 

,  sebaceous,  205 

,  subungual,  143 

,  submaxillary,  142 

,  sweat,  205 

,  thymus,  79,  85 

,  thyroid,  96 

Glisson's  capsule,  160 

Glomeruli  of  kidney,  211 

Glottis,  181 

Gluteal  muscles,  37 

Glycocholic  acid,  162 

Glycogen,  162,  167 

Glycosuria,  168 

Gmelin's  test  for  bile  pigments,  164 

Goblet  cells,  140 

Grooved  suture,  52 

Gustatory  cells,  212. 

H^.MAGLOBIX,   123,   124,   184 

Haematoidin,  164 

Hair,  204 

Hallux,  51 

Head,  movements  of,  23 

Hearmg,  255 

Heart,  79,  80,  11 3-1 20 

,  auricles  of,  81,  100.  117 

,  beats  of,  103 

,  grooves  of,  113 

,  movements  of,  102 

,  nerves  of,  72 

,  output  of,  104 

,  sounds  of,  103,  104 

,  ventricles  of,  81,  100,  11 7- 120 

,  work  of,  104 

Heat,  latent,  195 

,  loss  of,  194 

,  production  of,  194 

Hip-bone,  35 
Hippuric  acid,  217 
Homology  of  limb  bones,  37 
Humerus,  39 

,    Iliem,  90 
I    Ilium,  35 

Incisor  tooth,  31 


292 


Index. 


Incus,  257 

Innominate  artery,  82 
Inspiration,  34 
Intercostal  muscles,  33 
Internal  secretion,  96 
Intestine,  89-92 
Invertin,  145,  150 
Iris,  267 
Ischium,  35 

Jaw,  28 

,  movements  of,  28,  30 

,  muscles  of,  30 

Jejunum,  90 

Joints,  ball-and-socket,  37  ' 

,  different  kinds  of,  52-54 

Kidney,  blood-vessels  of,  212 

,  calix  of,  209 

— — ,  cortex  of,  211 

,  function  of,  214,  215 

,  glomeruli  of,  211 

,  Malpighian  corpuscles  of,  211 

,  medulla  of,  21 1 

,  pelvis  of,  204 

,  position  and  relations  of,  93 

,  structure  of,  209-212 

Knee-joint,  articulation  of,  48 
,  movements  of,  49 

Labyrinth  of  ear,  255,  258 

Lacteals,  91 

Larynx,  175 

Lecithine,  163 

Leucocytes,  2 

Levers,  three  orders  of,  74 

Ligaments,  17 

■ ,  orbicular,  42 

Limb  bones,  homology  of,  37 
Liver,  89,  158 

,  caudate  lobe  of,  89 

,  functions  of,  159,  165-172 

,  quadrate  lobe  of,  89 

,  spigelian  lobe  of,  89 

,  structure  of,  159-162 

Lobule  of  ear,  255 
Lungs,  85 

,  root  of,  85 

,  structure  of,  179,  180 

Lymph,  64 


Macula  lutea,  271 

Malleus,  256 

Mastication,  muscles  of,  30 

Mayer's  experiment,  274 

Medulla  oblongata,  223,  2_j2,  243, 

Membrana  tectoria,  259 

Membrane  basilar,  259 

of  Ressinus,  259 

Mesentery,  90,  91 
Meso-colon,  92 
Metacarpal  bones,  43,  44 
Metatarsal  bones,  51 
Modiolus,  259 
Molar  tooth,  31 

Motor  area,  224 

Movements,  co-ordinate,  238,  275 

of  elbow,  42 

of  fingers,  45. 

of  head, 23 

of  knee,  48 

of  v^rist,  42. 

,  reflex,  238 

Muscle,  56,  57 

,  cardiac,  68,  71 

,  chemical  stimulation  of,  69 

,  ciliary,  264 

,  contraction  of,  70 

,  co-ordination  of,  73 

,  curve,  70 

,  electrical  stimulation  of,  69 

,  excitation  of,  68 

,  involuntary,  61,  67,  69 

,  latent  period  of,  70 

,      mechanical     stimulation     of, 


69 


-,  nerve  ending  in,  67 

-,  nerve  of,  65 

-,  nutrition  of,  63-65 

-,  relaxation  of,  70 

-,  sense,  245 

-,  structure  of,  61-64 

-,  summation  of  stimuli,  71 

-,  tone  of,  60 

-,  voluntary  or  skeletal,  61,  62 


Muscles  of  eye,  264,  265 

,  gluteal,  37 

,  intercostal,  32,  33 

of  mastication,  30 

of  respiration,  65 

Musculi  papillaris,  116,  120 


Index. 


^93 


Xail  and  nail  bed,  204,  208 
Navicular  bone,  51 
Xerve,  afferent,  60,  226 

■  cells,  234 

-,  cranial,  227-231 

,  degeneration  of,  235 

,  efferent,  60,  227 

,  medullated  and  non-meduUated, 

233 
• ■  of  heart,  72 

roots,  226 

,  spinal,  226 

,  structure  of,  232-235 

-,  vasa  motor,  60,  in 

Nerves,  28,  281 
Nucleus,  2,  5 

Occipital  condyles,  23 
CEsophagus,  32,  79,  84 
Olfactory'  cells,  254 

membrane,  254 

Omentum,  89 

,  great,  90 

,  small,  90 

Optic  ner\'e,  266 

Os  calcis,  51 

Os  innominatum,  35 

Ovum,  segmentation  of,  10 

Oxy-haemoglobin,  184 

Palate,  hard,  28 

,  soft,  28 

Pancreas,  90 
Pancreatic  juice,  149 

— ,  action  of,  150 

,  composition  of,  150 

Patella,  48 
Pelvis,  35 

Pepsin,  148 

Peptone,  126 

Pericardium,  84 

Perilymph,  258 

Peripheral  resistance    to    blood,   10: 
T07 

Peristalsis,  60,  138 

Peritoneum,  88,  89 

Perspiration,  insensible,  207 

,  sensible,  207 

Pettenkofer's  test  for  bile  salts,  163 

Pharvnx,  28,  84 


Pinna,  255 
Pleura,  85,  86 
Pons  varolii,  223 
Portal  canal,  160 

vein,  157 

Pronation  of  hand,  42 
Proteids,  132,  134 

,  metabolism  of,  169 

Proteolytic  ferment,  145 
Protoplasm,  4,  34 

Ptyahn,  144-146 

Pubic  symphasis,  37 

Pubis,  35 

Pulmonary  circulation,  102 

veins,  114,  119 

Pulse,  1C7-109 

wave,  108 

Pupil,  266,  269 

Pyramidal  decussation,  224 

tracts,  224 

Quadrate  lobe  of  liver,  89 

Radius,  39 
I    Rectum,  60,  92 
I   Reflex  act,  237-239,  243 

Rennin  ferment,  145,  148 

Reserve  air,  181 

Residual  air,  180 

Respiration,  abdominal,  34 

,  costal,  34 

. ,  rate  of,  190,  191 

Respiratory  centre,  242 

Retina,  266,  269 

Ribs,  32 

Rolando,  fissure  of,  241 

Sacrum,  24 
Saliva,  7,  143,  144 

,  action  of,  146 

,  composition  of,  146 

Scala  tympani,  259 
Scaphoid  bone,  51 
Scapula,  37,  38 
Schneiderian  membrane,  254 
Sclerotic  coat,  265,  268 
Secretion,  internal,  96 

of  urine,  213 

Semicircular  canals,  259,  263 
Semilunar  fibro-cartilages,  48 


294 


Index. 


Sensation,  common,  245; 

,  cutaneous,  245 

— — ,  special,  245 
Sanse,  muscular,  245 

of  hearing,  255 

•  of  smell,  252,  254 

of  taste,  252  -254 

of  touch,  246 

Serum,  128,  129 
Sesamoid  bone,  48 
Shoulder,  articulation  of,  39,  40 
Simultaneous  contrast,  274 
Skeletal  movements,  t^,  75,  76 
Skin,  appendages  of,  203 

,  structure  of,  200-202 

Skull,  24-26 

Special  sensation,  law  of,  247 
Spermatozoon,  11 
Sphygmograph,  109 
Spigelian  lobe  of  liver,  89 
Spinal  cord,  225 

,  central  canal,  226 

centres,  244 

tracts,  236 

nerves,  226 

Spleen,  92,  93 
Stapes,  257 

Steatolytic  ferment,  145 
Stercobilin,  164 
Stomach,  89,  90,  139 

,  movements  of,  138,  148 

,  mucous  membrane  of,  146,  147 

,  structure  of,  138 

Subjective  sensation,  247,  250,  251 
Successive  contrast,  274 
Succus  entericus,  T50 
Sulci  of  brain,  219 
Supination  of  hand,  42 
Supplementary  air,  181 
Suprarenal  bodies,  94,  96 
Suspensory  ligament  of  lens,  266 
Sutures,  25,  52 
Sweat,  composition  of,  207 

glands,  205,  206 

Sympathetic   system,    219,    231,   236- 

237 
Symphasis  pubis,  52 
Synarthrosis,  52 
Synchondrosis,  53 
Synovial  fluid,  16 


Tarsus,  50,  51 
Taste,  252 
Taurine,  162 
Taurocholic  acid,  162 
Temperature  of  animals,  192 

sense,  250 

Tendo  Achillis,  51 
Tendon,  56,  63 
Tetanus,  71 
Thoracic  duct,  65,  79 
Thorax,  movements  of,  31-34 

■,  structure  of,  31,  32 

Thymus  gland,  79,  85 

Thyroid  gland,  96 

Tibia,  50 

Tidal  air,  181 

"  Tissue,"  definition  of,  6 

Tone  of  muscle,  60 

Tooth,  bicuspid,  31 

,  canine,  31 

,  incisor,  31 

,  molar,  31 

Trachea,  174,  175 

,  structure  of,  176 

Trochanter,  45,  46 
Trochlea,  40 
Trypsin,  149 

Tympanic  membrane,  256 
Tympanum,  255,  256 

Ulna,  39,  213,  216 

Urea,  165,  216 

Ureter,  93 

Urethra,  94 

Uric  acid,  216 

Urine,  composition  of,  215,  217 

,  secretion  of,  213,  214 

Uriniferous  tubules,  211 

Valve,  aortic,  120 

,  auriculo-ventricular,  115 

,  ileo-cascal,  91 

,  mitral,  119 

,  pulmonary,  119 

,  semi-lunar,  119 

,  tricuspid,  120 

of  veins,  iii,  112 

Valvulas  conniventes,  155 
Vasa  motor  centre,  243 


Index. 


295 


Vasa  motor  nerves,  60,  iii 
Vein,  portal,  157 

,  pulmonary,  114,  119 

,  structure  of,  99 

Velocity  of  blood,  109,  no 

of  pulse,  109 

Velum,  28 

Vena  cava,  inferior,  81,  114,  159 

,  superior,  81 

Ventricles  of  brain,  221 

of  heart,  100,  117-120. 

Vermiform  appendix,  91 
Vertebra,  cervical,  21 


Vertebra,  cocygeal,  24 

,  dorsal  or  thoracic,  23 

,  lumVjar,  24 

,  sacral,  24 

Vertebral  column,  17,  18 
Vestibule  of  ear,  259 
Villi  of  intestine,  152-155 
Vital  capacity,  181 
Vitreous  humour,  267 

Warm-blooded  animals,  192,  193 
Water,  excretion  of,  198 
Weber-Feehner  law,  248 


PRINTED    BY   WILLIAM    CLOWES  AND   SONS,    LIMllfci),    LONUOX    ANU    BECCLES. 


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